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2.4: Chapter 5 - What Is a Search?- Wrapping Up

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    For Chapter 3, we read Smith v. Maryland, in which the Court decided in 1979 that installation and use of a “pen register” to learn what numbers a suspect called from his home telephone was not a Fourth Amendment “search.” Nearly 40 years later, the Court considered whether the holding of Smith allowed the government to gather a suspect’s cell phone records to learn where that suspect has been. The question sharply divided the Court. Chief Justice Roberts wrote for a five-Justice majority. Each Justice who dissented wrote his own dissenting opinion. The dissents and the majority opinion combined to fill 119 pages in the Court’s slip opinion.

    Supreme Court of the United States

    Timothy Carpenter v. United States

    Decided June 22, 2018 – 138 S. Ct. 2206

    Chief Justice ROBERTS delivered the opinion of the Court.

    This case presents the question whether the Government conducts a search under the Fourth Amendment when it accesses historical cell phone records that provide a comprehensive chronicle of the user’s past movements.


    There are 396 million cell phone service accounts in the United States—for a Nation of 326 million people. Cell phones perform their wide and growing variety of functions by connecting to a set of radio antennas called “cell sites.” Although cell sites are usually mounted on a tower, they can also be found on light posts, flagpoles, church steeples, or the sides of buildings. Cell sites typically have several directional antennas that divide the covered area into sectors.

    Cell phones continuously scan their environment looking for the best signal, which generally comes from the closest cell site. Most modern devices, such as smartphones, tap into the wireless network several times a minute whenever their signal is on, even if the owner is not using one of the phone’s features. Each time the phone connects to a cell site, it generates a time-stamped record known as cell-site location information (CSLI). The precision of this information depends on the size of the geographic area covered by the cell site. The greater the concentration of cell sites, the smaller the coverage area. As data usage from cell phones has increased, wireless carriers have installed more cell sites to handle the traffic. That has led to increasingly compact coverage areas, especially in urban areas.

    Wireless carriers collect and store CSLI for their own business purposes, including finding weak spots in their network and applying “roaming” charges when another carrier routes data through their cell sites. In addition, wireless carriers often sell aggregated location records to data brokers, without individual identifying information of the sort at issue here. While carriers have long retained CSLI for the start and end of incoming calls, in recent years phone companies have also collected location information from the transmission of text messages and routine data connections. Accordingly, modern cell phones generate increasingly vast amounts of increasingly precise CSLI.


    In 2011, police officers arrested four men suspected of robbing a series of Radio Shack and (ironically enough) T–Mobile stores in Detroit. One of the men confessed that, over the previous four months, the group (along with a rotating cast of getaway drivers and lookouts) had robbed nine different stores in Michigan and Ohio. The suspect identified 15 accomplices who had participated in the heists and gave the FBI some of their cell phone numbers; the FBI then reviewed his call records to identify additional numbers that he had called around the time of the robberies.

    Based on that information, the prosecutors applied for court orders under the Stored Communications Act to obtain cell phone records for petitioner Timothy Carpenter and several other suspects. That statute, as amended in 1994, permits the Government to compel the disclosure of certain telecommunications records when it “offers specific and articulable facts showing that there are reasonable grounds to believe” that the records sought “are relevant and material to an ongoing criminal investigation.” Federal Magistrate Judges issued two orders directing Carpenter’s wireless carriers—MetroPCS and Sprint—to disclose “cell/site sector [information] for [Carpenter’s] telephone[ ] at call origination and at call termination for incoming and outgoing calls” during the four-month period when the string of robberies occurred. The first order sought 152 days of cell-site records from MetroPCS, which produced records spanning 127 days. The second order requested seven days of CSLI from Sprint, which produced two days of records covering the period when Carpenter’s phone was “roaming” in northeastern Ohio. Altogether the Government obtained 12,898 location points cataloging Carpenter’s movements—an average of 101 data points per day.

    Carpenter was charged with six counts of robbery and an additional six counts of carrying a firearm during a federal crime of violence. Prior to trial, Carpenter moved to suppress the cell-site data provided by the wireless carriers. He argued that the Government’s seizure of the records violated the Fourth Amendment because they had been obtained without a warrant supported by probable cause. The District Court denied the motion.

    At trial, seven of Carpenter’s confederates pegged him as the leader of the operation. In addition, FBI agent Christopher Hess offered expert testimony about the cell-site data. Hess explained that each time a cell phone taps into the wireless network, the carrier logs a time-stamped record of the cell site and particular sector that were used. With this information, Hess produced maps that placed Carpenter’s phone near four of the charged robberies. In the Government’s view, the location records clinched the case: They confirmed that Carpenter was “right where the … robbery was at the exact time of the robbery.” Carpenter was convicted on all but one of the firearm counts and sentenced to more than 100 years in prison.

    The Court of Appeals for the Sixth Circuit affirmed. The court held that Carpenter lacked a reasonable expectation of privacy in the location information collected by the FBI because he had shared that information with his wireless carriers. Given that cell phone users voluntarily convey cell-site data to their carriers as “a means of establishing communication,” the court concluded that the resulting business records are not entitled to Fourth Amendment protection. We granted certiorari.


    The Founding generation crafted the Fourth Amendment as a “response to the reviled ‘general warrants’ and ‘writs of assistance’ of the colonial era, which allowed British officers to rummage through homes in an unrestrained search for evidence of criminal activity.” In fact, as John Adams recalled, the patriot James Otis’s 1761 speech condemning writs of assistance was “the first act of opposition to the arbitrary claims of Great Britain” and helped spark the Revolution itself.

    For much of our history, Fourth Amendment search doctrine was “tied to common-law trespass” and focused on whether the Government “obtains information by physically intruding on a constitutionally protected area.” More recently, the Court has recognized that “property rights are not the sole measure of Fourth Amendment violations.” In Katz v. United States, we established that “the Fourth Amendment protects people, not places,” and expanded our conception of the Amendment to protect certain expectations of privacy as well.

    Although no single rubric definitively resolves which expectations of privacy are entitled to protection, the analysis is informed by historical understandings “of what was deemed an unreasonable search and seizure when [the Fourth Amendment] was adopted.” On this score, our cases have recognized some basic guideposts. First, that the Amendment seeks to secure “the privacies of life” against “arbitrary power.” Second, and relatedly, that a central aim of the Framers was “to place obstacles in the way of a too permeating police surveillance.”

    We have kept this attention to Founding-era understandings in mind when applying the Fourth Amendment to innovations in surveillance tools. As technology has enhanced the Government’s capacity to encroach upon areas normally guarded from inquisitive eyes, this Court has sought to “assure[ ] preservation of that degree of privacy against government that existed when the Fourth Amendment was adopted.”


    The case before us involves the Government’s acquisition of wireless carrier cell-site records revealing the location of Carpenter’s cell phone whenever it made or received calls. This sort of digital data—personal location information maintained by a third party—does not fit neatly under existing precedents. Instead, requests for cell-site records lie at the intersection of two lines of cases, both of which inform our understanding of the privacy interests at stake.

    The first set of cases addresses a person’s expectation of privacy in his physical location and movements. The Court [has] concluded that “augment[ed]” visual surveillance [does] not constitute a search because “[a] person traveling in an automobile on public thoroughfares has no reasonable expectation of privacy in his movements from one place to another.” Since the movements of the vehicle and its final destination [are] “voluntarily conveyed to anyone who wanted to look,” [defendant] could not assert a privacy interest in the information obtained.

    In a second set of decisions, the Court has drawn a line between what a person keeps to himself and what he shares with others. We have previously held that “a person has no legitimate expectation of privacy in information he voluntarily turns over to third parties.” That remains true “even if the information is revealed on the assumption that it will be used only for a limited purpose.” As a result, the Government is typically free to obtain such information from the recipient without triggering Fourth Amendment protections.


    The question we confront today is how to apply the Fourth Amendment to a new phenomenon: the ability to chronicle a person’s past movements through the record of his cell phone signals. Such tracking partakes of many of the qualities of the GPS monitoring we considered in Jones. Much like GPS tracking of a vehicle, cell phone location information is detailed, encyclopedic, and effortlessly compiled.

    At the same time, the fact that the individual continuously reveals his location to his wireless carrier implicates the third-party principle. But while the third-party doctrine applies to telephone numbers and bank records, it is not clear whether its logic extends to the qualitatively different category of cell-site records. After all, when Smith was decided in 1979, few could have imagined a society in which a phone goes wherever its owner goes, conveying to the wireless carrier not just dialed digits, but a detailed and comprehensive record of the person’s movements.

    We decline to extend [the third-party principle] to cover these novel circumstances. Given the unique nature of cell phone location records, the fact that the information is held by a third party does not by itself overcome the user’s claim to Fourth Amendment protection. Whether the Government employs its own surveillance technology or leverages the technology of a wireless carrier, we hold that an individual maintains a legitimate expectation of privacy in the record of his physical movements as captured through CSLI. The location information obtained from Carpenter’s wireless carriers was the product of a search.


    A person does not surrender all Fourth Amendment protection by venturing into the public sphere. To the contrary, “what [one] seeks to preserve as private, even in an area accessible to the public, may be constitutionally protected.” Prior to the digital age, law enforcement might have pursued a suspect for a brief stretch, but doing so “for any extended period of time was difficult and costly and therefore rarely undertaken.” For that reason, “society’s expectation has been that law enforcement agents and others would not—and indeed, in the main, simply could not—secretly monitor and catalogue every single movement of an individual’s car for a very long period.”

    Allowing government access to cell-site records contravenes that expectation. Although such records are generated for commercial purposes, that distinction does not negate Carpenter’s anticipation of privacy in his physical location. Mapping a cell phone’s location over the course of 127 days provides an all-encompassing record of the holder’s whereabouts. As with GPS information, the time-stamped data provides an intimate window into a person’s life, revealing not only his particular movements, but through them his “familial, political, professional, religious, and sexual associations.” These location records “hold for many Americans the ‘privacies of life.’” And like GPS monitoring, cell phone tracking is remarkably easy, cheap, and efficient compared to traditional investigative tools. With just the click of a button, the Government can access each carrier’s deep repository of historical location information at practically no expense.

    In fact, historical cell-site records present even greater privacy concerns than the GPS monitoring of a vehicle we considered in Jones. Unlike the bugged container in Knotts or the car in Jones, a cell phone—almost a “feature of human anatomy,”—tracks nearly exactly the movements of its owner. While individuals regularly leave their vehicles, they compulsively carry cell phones with them all the time. A cell phone faithfully follows its owner beyond public thoroughfares and into private residences, doctor’s offices, political headquarters, and other potentially revealing locales. Accordingly, when the Government tracks the location of a cell phone it achieves near perfect surveillance, as if it had attached an ankle monitor to the phone’s user.

    Moreover, the retrospective quality of the data here gives police access to a category of information otherwise unknowable. In the past, attempts to reconstruct a person’s movements were limited by a dearth of records and the frailties of recollection. With access to CSLI, the Government can now travel back in time to retrace a person’s whereabouts, subject only to the retention [policies] of the wireless carriers, which currently maintain records for up to five years. Critically, because location information is continually logged for all of the 400 million devices in the United States—not just those belonging to persons who might happen to come under investigation—this newfound tracking capacity runs against everyone. Unlike with the GPS device in Jones, police need not even know in advance whether they want to follow a particular individual, or when.

    Whoever the suspect turns out to be, he has effectively been tailed every moment of every day for five years, and the police may—in the Government’s view—call upon the results of that surveillance without regard to the constraints of the Fourth Amendment. Only the few without cell phones could escape this tireless and absolute surveillance.

    Accordingly, when the Government accessed CSLI from the wireless carriers, it invaded Carpenter’s reasonable expectation of privacy in the whole of his physical movements.


    The Government’s primary contention to the contrary is that the third-party doctrine governs this case. In its view, cell-site records are fair game because they are “business records” created and maintained by the wireless carriers. The Government recognizes that this case features new technology, but asserts that the legal question nonetheless turns on a garden-variety request for information from a third-party witness.

    The Government’s position fails to contend with the seismic shifts in digital technology that made possible the tracking of not only Carpenter’s location but also everyone else’s, not for a short period but for years and years. Sprint Corporation and its competitors are not your typical witnesses. Unlike the nosy neighbor who keeps an eye on comings and goings, they are ever alert, and their memory is nearly infallible. There is a world of difference between limited types of personal information and the exhaustive chronicle of location information casually collected by wireless carriers today. The Government thus is not asking for a straightforward application of the third-party doctrine, but instead a significant extension of it to a distinct category of information.

    The third-party doctrine partly stems from the notion that an individual has a reduced expectation of privacy in information knowingly shared with another. But the fact of “diminished privacy interests does not mean that the Fourth Amendment falls out of the picture entirely.”

    Neither does the second rationale underlying the third-party doctrine—voluntary exposure—hold up when it comes to CSLI. Cell phone location information is not truly “shared” as one normally understands the term. In the first place, cell phones and the services they provide are “such a pervasive and insistent part of daily life” that carrying one is indispensable to participation in modern society. Second, a cell phone logs a cell-site record by dint of its operation, without any affirmative act on the part of the user beyond powering up. Virtually any activity on the phone generates CSLI, including incoming calls, texts, or e-mails and countless other data connections that a phone automatically makes when checking for news, weather, or social media updates. Apart from disconnecting the phone from the network, there is no way to avoid leaving behind a trail of location data. As a result, in no meaningful sense does the user voluntarily “assume[ ] the risk” of turning over a comprehensive dossier of his physical movements.

    Given the unique nature of cell phone location information, the fact that the Government obtained the information from a third party does not overcome Carpenter’s claim to Fourth Amendment protection. The Government’s acquisition of the cell-site records was a search within the meaning of the Fourth Amendment.

    Our decision today is a narrow one. We do not express a view on matters not before us: real-time CSLI or “tower dumps” (a download of information on all the devices that connected to a particular cell site during a particular interval). We do not call into question conventional surveillance techniques and tools, such as security cameras. Nor do we address other business records that might incidentally reveal location information. Further, our opinion does not consider other collection techniques involving foreign affairs or national security. As Justice Frankfurter noted when considering new innovations in airplanes and radios, the Court must tread carefully in such cases, to ensure that we do not “embarrass the future.”

    We decline to grant the state unrestricted access to a wireless carrier’s database of physical location information. In light of the deeply revealing nature of CSLI, its depth, breadth, and comprehensive reach, and the inescapable and automatic nature of its collection, the fact that such information is gathered by a third party does not make it any less deserving of Fourth Amendment protection. The Government’s acquisition of the cell-site records here was a search under that Amendment.

    The judgment of the Court of Appeals is reversed, and the case is remanded for further proceedings consistent with this opinion.

    Justice KENNEDY, with whom Justice THOMAS and Justice ALITO join, dissenting.

    This case involves new technology, but the Court’s stark departure from relevant Fourth Amendment precedents and principles is, in my submission, unnecessary and incorrect, requiring this respectful dissent.

    The new rule the Court seems to formulate puts needed, reasonable, accepted, lawful, and congressionally authorized criminal investigations at serious risk in serious cases, often when law enforcement seeks to prevent the threat of violent crimes. And it places undue restrictions on the lawful and necessary enforcement powers exercised not only by the Federal Government, but also by law enforcement in every State and locality throughout the Nation. Adherence to this Court’s longstanding precedents and analytic framework would have been the proper and prudent way to resolve this case.

    The Court has twice held that individuals have no Fourth Amendment interests in business records which are possessed, owned, and controlled by a third party. This is true even when the records contain personal and sensitive information. So when the Government uses a subpoena to obtain, for example, bank records, telephone records, and credit card statements from the businesses that create and keep these records, the Government does not engage in a search of the business’s customers within the meaning of the Fourth Amendment.

    In this case petitioner challenges the Government’s right to use compulsory process to obtain a now-common kind of business record: cell-site records held by cell phone service providers. The Government acquired the records through an investigative process enacted by Congress. Upon approval by a neutral magistrate, and based on the Government’s duty to show reasonable necessity, it authorizes the disclosure of records and information that are under the control and ownership of the cell phone service provider, not its customer.

    Cell-site records are no different from the many other kinds of business records the Government has a lawful right to obtain by compulsory process. Customers like petitioner do not own, possess, control, or use the records, and for that reason have no reasonable expectation that they cannot be disclosed pursuant to lawful compulsory process.

    In concluding that the Government engaged in a search, the Court unhinges Fourth Amendment doctrine from the property-based concepts that have long grounded the analytic framework that pertains in these cases. In doing so it draws an unprincipled and unworkable line between cell-site records on the one hand and financial and telephonic records on the other. According to today’s majority opinion, the Government can acquire a record of every credit card purchase and phone call a person makes over months or years without upsetting a legitimate expectation of privacy. But, in the Court’s view, the Government crosses a constitutional line when it obtains a court’s approval to issue a subpoena for more than six days of cell-site records in order to determine whether a person was within several hundred city blocks of a crime scene. That distinction is illogical and will frustrate principled application of the Fourth Amendment in many routine yet vital law enforcement operations.

    Justice THOMAS, dissenting.

    [Justice Thomas raised two primary arguments in his dissent. First, he noted that the “property” at issue belonged to MetroPCS and Sprint, and that Carpenter accordingly had no ground upon which to object to a search of the property. Second, he argued that the Court should reject entirely the “reasonable expectation of privacy” test, which Justice Thomas wrote has served “to distort Fourth Amendment jurisprudence.”]

    Justice ALITO, with whom Justice THOMAS joins, dissenting.

    I share the Court’s concern about the effect of new technology on personal privacy, but I fear that today’s decision will do far more harm than good. The Court’s reasoning fractures two fundamental pillars of Fourth Amendment law, and in doing so, it guarantees a blizzard of litigation while threatening many legitimate and valuable investigative practices upon which law enforcement has rightfully come to rely.

    First, the Court ignores the basic distinction between an actual search (dispatching law enforcement officers to enter private premises and root through private papers and effects) and an order merely requiring a party to look through its own records and produce specified documents. The former, which intrudes on personal privacy far more deeply, requires probable cause; the latter does not. Treating an order to produce like an actual search, as today’s decision does, is revolutionary. It violates both the original understanding of the Fourth Amendment and more than a century of Supreme Court precedent. Unless it is somehow restricted to the particular situation in the present case, the Court’s move will cause upheaval. Must every grand jury subpoena duces tecum be supported by probable cause? If so, investigations of terrorism, political corruption, white-collar crime, and many other offenses will be stymied. And what about subpoenas and other document-production orders issued by administrative agencies?

    Second, the Court allows a defendant to object to the search of a third party’s property. This also is revolutionary. The Fourth Amendment protects “[t]he right of the people to be secure in their persons, houses, papers, and effects” (emphasis added), not the persons, houses, papers, and effects of others. Until today, we have been careful to heed this fundamental feature of the Amendment’s text.

    By departing dramatically from these fundamental principles, the Court destabilizes long-established Fourth Amendment doctrine. We will be making repairs—or picking up the pieces—for a long time to come.

    Although the majority professes a desire not to “‘embarrass the future,’” we can guess where today’s decision will lead.

    One possibility is that the broad principles that the Court seems to embrace will be applied across the board. All subpoenas duces tecum and all other orders compelling the production of documents will require a demonstration of probable cause, and individuals will be able to claim a protected Fourth Amendment interest in any sensitive personal information about them that is collected and owned by third parties. Those would be revolutionary developments indeed.

    The other possibility is that this Court will face the embarrassment of explaining in case after case that the principles on which today’s decision rests are subject to all sorts of qualifications and limitations that have not yet been discovered. If we take this latter course, we will inevitably end up “mak[ing] a crazy quilt of the Fourth Amendment.”

    The desire to make a statement about privacy in the digital age does not justify the consequences that today’s decision is likely to produce.

    Justice GORSUCH, dissenting.

    [Justice Gorsuch echoed some of the arguments raised by Justice Thomas concerning the wisdom of the “reasonable expectation of privacy” test. He then suggested that Carpenter might have prevailed on a different theory, based on the trespass test reinvigorated by United States v. Jones (2012) and Florida v. Jardines, 133 S. Ct. 1409 (2013) (part of the reading for our next chapter). Under this theory, perhaps Carpenter had standing to object to a search of property held by MetroPCS and Sprint. One often retains rights to property deposited with a third party; recall the concept of a “bailment.” Because Justice Gorsuch “reluctantly” concluded that Carpenter “forfeited perhaps his most promising line of argument” by not raising it, Justice Gorsuch could not concur in the judgment (in favor of Carpenter) and instead dissented.]

    Notes, Comments, and Questions

    In Chapter 2, students were encouraged to consider how the doctrines set forth in Katz and Jones would resolve the question presented in Carpenter. Now, having read Carpenter, students should review their analysis. Did the Court reach the result you expected? If not, why do you think the Court’s reasoning differed from yours?

    As Justice Sotomayor noted in her concurrence in United States v. Jones (Chapter 2), technological advances will demand continued attention from the Court. Students should also consider when and how the legislative and executive branches of the federal government (as well as the states) should regulate privacy related to smart phones and other technological marvels.

    In many cities, drivers can use an app called “ParkMobile” to pay for parking, saving them the trouble of finding change for parking meters. The app allows users to look up their parking “history,” which is a list of times and places of prior transactions. Imagine that after a bank robbery, police contact ParkMobile corporate headquarters and obtain a list of all users who parked within a mile of the bank within an hour of the robbery. Search or no search? Why or why not? Now, imagine instead that police suspect a particular person of robbing the bank, and police contact ParkMobile to obtain that single person’s parking history for the day of the robbery. Search or no search? Why or why not?

    Consider this commentary by Evan Caminker, a professor at and former dean of the University of Michigan Law School. Caminker briefed and argued Carpenter on behalf of the United States when the case was at the Sixth Circuit; he was on academic leave and was working with the federal prosecutors in Michigan. After Caminker was back in academia—and the Supreme Court had decided the case—he published these reflections:

    How should courts square Katz in the future? The Court in Carpenter said there are two separate rationales underlying the third-party doctrine—lack of special sensitivity and voluntary exposure—and that CSLI triggers neither. As with its involuntary arguments, however, the Court does not explain how the two rationales relate as part of the overall doctrine.

    The Court might mean that the third-party doctrine applies when either of the two rationales is present. In other words, if highly sensitive information was voluntarily conveyed (think Fitbit health data), or if nonsensitive information was involuntarily shared (perhaps computer internet protocol addresses?), then the privacy interest dissipates. This reading fits with the Court’s decision to address both variables, rather than to end its analysis after finding no voluntary sharing.

    Or, the Court might mean that privacy dissipates only if both rationales apply, and the information is both voluntarily shared and nonsensitive. That seems perfectly logical too, though it seems less likely because it would mean that the third-party doctrine can never apply to sensitive information, no matter how clearly it was voluntarily shared (think of Carpenter posting his own location history on Facebook, or celebrities publishing tell-all memoirs).

    And then there is a third possibility, raised and criticized by the dissents: an open-ended multifactor test. Justice Gorsuch, for example, lamented a “second Katz-like balancing inquiry, asking whether the fact of disclosure … outweighs privacy interests in the ‘category of information’ so disclosed.” Justice Kennedy also viewed the Court as announcing a balancing test that encompassed both privacy interests and CSLI tracking properties by “considering intimacy, comprehensiveness, expense, retrospectivity, and voluntariness.” Of course, the Court often articulates doctrine through multifactor tests, but Justice Kennedy feared that this one would particularly put “the law on a new and unstable foundation” as lower courts would be left to figure out for themselves how the doctrinal variables relate when they address other surveillance technologies and types of digital data.

    Evan Caminker, Location Tracking and Digital Data: Can Carpenter Build A Stable Privacy Doctrine?, 2018 S. Ct. Rev. 411, 450–51 (2018).

    Less than two years after Carpenter was decided, reporters revealed that federal agents had “bought access to a commercial database that maps the movements of millions of cellphones in America” and were “using it for immigration and border enforcement.” See Bryan Tau & Michelle Hackman, Federal Agencies Use Cellphone Location Data for Immigration Enforcement, Wall St. J. (Feb. 7, 2020). Asked about whether Fourth Amendment law might regulate this tactic, a former Homeland Security official said, “In this case, the government is a commercial purchaser like anybody else. Carpenter is not relevant.” He added, “The government is just buying a widget.”

    * * *

    In our remaining material for this chapter, we will see how the Court has applied its Fourth Amendment principles to a more old-fashioned investigatory tool: the use of dogs by police. Depending on the context—a dog sniffing bags at an airport, a dog sniffing a car during a traffic stop, a dog sniffing someone’s porch—the Court has reached different conclusions on whether using a dog is a “search.”

    Supreme Court of the United States

    United States v. Raymond J. Place

    Decided June 20, 1983 – 462 U.S. 696

    Justice O’CONNOR delivered the opinion of the Court.

    Respondent Raymond J. Place’s behavior aroused the suspicions of law enforcement officers as he waited in line at the Miami International Airport to purchase a ticket to New York’s LaGuardia Airport. As Place proceeded to the gate for his flight, the agents approached him and requested his airline ticket and some identification. Place complied with the request and consented to a search of the two suitcases he had checked. Because his flight was about to depart, however, the agents decided not to search the luggage.

    Prompted by Place’s parting remark that he had recognized that they were police, the agents inspected the address tags on the checked luggage and noted discrepancies in the two street addresses. Further investigation revealed that neither address existed and that the telephone number Place had given the airline belonged to a third address on the same street. On the basis of their encounter with Place and this information, the Miami agents called Drug Enforcement Administration (DEA) authorities in New York to relay their information about Place.

    Two DEA agents waited for Place at the arrival gate at LaGuardia Airport in New York. There again, his behavior aroused the suspicion of the agents. After he had claimed his two bags and called a limousine, the agents decided to approach him. They identified themselves as federal narcotics agents, to which Place responded that he knew they were “cops” and had spotted them as soon as he had deplaned. One of the agents informed Place that, based on their own observations and information obtained from the Miami authorities, they believed that he might be carrying narcotics. After identifying the bags as belonging to him, Place stated that a number of police at the Miami Airport had surrounded him and searched his baggage. The agents responded that their information was to the contrary. The agents requested and received identification from Place—a New Jersey driver’s license, on which the agents later ran a computer check that disclosed no offenses, and his airline ticket receipt. When Place refused to consent to a search of his luggage, one of the agents told him that they were going to take the luggage to a federal judge to try to obtain a search warrant and that Place was free to accompany them. Place declined, but obtained from one of the agents telephone numbers at which the agents could be reached.

    The agents then took the bags to Kennedy Airport, where they subjected the bags to a “sniff test” by a trained narcotics detection dog. The dog reacted positively to the smaller of the two bags but ambiguously to the larger bag. Because it was late on a Friday afternoon, the agents retained the luggage until Monday morning, when they secured a search warrant from a magistrate for the smaller bag. Upon opening that bag, the agents discovered 1,125 grams of cocaine.

    Place was indicted for possession of cocaine with intent to distribute. In the District Court, Place moved to suppress the contents of the luggage seized from him at LaGuardia Airport, claiming that the warrantless seizure of the luggage violated his Fourth Amendment rights. The District Court denied the motion.

    On appeal of the conviction, the United States Court of Appeals for the Second Circuit reversed. We granted certiorari and now affirm.

    The purpose for which respondent’s luggage was seized was to arrange its exposure to a narcotics detection dog. Obviously, if this investigative procedure is itself a search requiring probable cause, the initial seizure of respondent’s luggage for the purpose of subjecting it to the sniff test—no matter how brief—could not be justified on less than probable cause.

    The Fourth Amendment “protects people from unreasonable government intrusions into their legitimate expectations of privacy.” We have affirmed that a person possesses a privacy interest in the contents of personal luggage that is protected by the Fourth Amendment. A “canine sniff” by a well-trained narcotics detection dog, however, does not require opening the luggage. It does not expose noncontraband items that otherwise would remain hidden from public view, as does, for example, an officer’s rummaging through the contents of the luggage. Thus, the manner in which information is obtained through this investigative technique is much less intrusive than a typical search. Moreover, the sniff discloses only the presence or absence of narcotics, a contraband item. Thus, despite the fact that the sniff tells the authorities something about the contents of the luggage, the information obtained is limited. This limited disclosure also ensures that the owner of the property is not subjected to the embarrassment and inconvenience entailed in less discriminate and more intrusive investigative methods.

    In these respects, the canine sniff is sui generis. We are aware of no other investigative procedure that is so limited both in the manner in which the information is obtained and in the content of the information revealed by the procedure. Therefore, we conclude that the particular course of investigation that the agents intended to pursue here—exposure of respondent’s luggage, which was located in a public place, to a trained canine—did not constitute a “search” within the meaning of the Fourth Amendment.

    [Although the Court found that the dog sniff was not a “search,” Place prevailed because the Court held that police committed an unlawful seizure of Place’s property by detaining his luggage for 90 minutes and not informing him of their plans for the luggage. The concurring opinion below disagrees with the majority’s conclusion about the dog sniff.]

    Justice BRENNAN, with whom Justice MARSHALL joins, concurring in the result.

    The Court suggests today that exposure of respondent’s luggage to a narcotics detection dog “did not constitute a ‘search’ within the meaning of the Fourth Amendment.”

    [T]he issue is more complex than the Court’s discussion would lead one to believe. As Justice STEVENS suggested in objecting to “unnecessarily broad dicta” in United States v. Knotts, the use of electronic detection techniques that enhance human perception implicates “especially sensitive concerns.” Obviously, a narcotics detection dog is not an electronic detection device. Unlike the electronic “beeper” in Knotts, however, a dog does more than merely allow the police to do more efficiently what they could do using only their own senses. A dog adds a new and previously unobtainable dimension to human perception. The use of dogs, therefore, represents a greater intrusion into an individual’s privacy. Such use implicates concerns that are at least as sensitive as those implicated by the use of certain electronic detection devices.

    I have expressed the view that dog sniffs of people constitute searches. In any event, I would leave the determination of whether dog sniffs of luggage amount to searches, and the subsidiary question of what standards should govern such intrusions, to a future case providing an appropriate, and more informed, basis for deciding these questions.

    * * *

    In Place, the Court focused on the use of dogs in an airport, which is a public place that persons visit by choice. In the next case, the Court turned its attention to the use of dogs during traffic stops, in which motorists are detained involuntarily.

    Supreme Court of the United States

    Illinois v. Roy I. Caballes

    Decided Jan. 24, 2005 – 543 U.S. 405

    Justice STEVENS delivered the opinion of the Court.

    Illinois State Trooper Daniel Gillette stopped respondent for speeding on an interstate highway. When Gillette radioed the police dispatcher to report the stop, a second trooper, Craig Graham, a member of the Illinois State Police Drug Interdiction Team, overheard the transmission and immediately headed for the scene with his narcotics-detection dog. When they arrived, respondent’s car was on the shoulder of the road and respondent was in Gillette’s vehicle. While Gillette was in the process of writing a warning ticket, Graham walked his dog around respondent’s car. The dog alerted at the trunk. Based on that alert, the officers searched the trunk, found marijuana, and arrested respondent. The entire incident lasted less than 10 minutes.

    Respondent was convicted of a narcotics offense and sentenced to 12 years’ imprisonment and a $256,136 fine. The trial judge denied his motion to suppress the seized evidence and to quash his arrest. He held that the officers had not unnecessarily prolonged the stop and that the dog alert was sufficiently reliable to provide probable cause to conduct the search. Although the Appellate Court affirmed, the Illinois Supreme Court reversed, concluding that because the canine sniff was performed without any “‘specific and articulable facts’” to suggest drug activity, the use of the dog “unjustifiably enlarg[ed] the scope of a routine traffic stop into a drug investigation.”

    The question on which we granted certiorari is narrow: “Whether the Fourth Amendment requires reasonable, articulable suspicion to justify using a drug-detection dog to sniff a vehicle during a legitimate traffic stop.” Thus, we proceed on the assumption that the officer conducting the dog sniff had no information about respondent except that he had been stopped for speeding; accordingly, we have omitted any reference to facts about respondent that might have triggered a modicum of suspicion.

    [T]he Illinois Supreme Court held that the initially lawful traffic stop became an unlawful seizure solely as a result of the canine sniff that occurred outside respondent’s stopped car. That is, the court characterized the dog sniff as the cause rather than the consequence of a constitutional violation. In its view, the use of the dog converted the citizen-police encounter from a lawful traffic stop into a drug investigation, and because the shift in purpose was not supported by any reasonable suspicion that respondent possessed narcotics, it was unlawful. In our view, conducting a dog sniff would not change the character of a traffic stop that is lawful at its inception and otherwise executed in a reasonable manner, unless the dog sniff itself infringed respondent’s constitutionally protected interest in privacy. Our cases hold that it did not.

    Official conduct that does not “compromise any legitimate interest in privacy” is not a search subject to the Fourth Amendment. We have held that any interest in possessing contraband cannot be deemed “legitimate,” and thus, governmental conduct that only reveals the possession of contraband “compromises no legitimate privacy interest.” This is because the expectation “that certain facts will not come to the attention of the authorities” is not the same as an interest in “privacy that society is prepared to consider reasonable.” Respondent concedes that “drug sniffs are designed, and if properly conducted are generally likely, to reveal only the presence of contraband.” Although respondent argues that the error rates, particularly the existence of false positives, call into question the premise that drug-detection dogs alert only to contraband, the record contains no evidence or findings that support his argument. Moreover, respondent does not suggest that an erroneous alert, in and of itself, reveals any legitimate private information, and, in this case, the trial judge found that the dog sniff was sufficiently reliable to establish probable cause to conduct a full-blown search of the trunk.

    Accordingly, the use of a well-trained narcotics-detection dog—one that “does not expose noncontraband items that otherwise would remain hidden from public view,” during a lawful traffic stop, generally does not implicate legitimate privacy interests. In this case, the dog sniff was performed on the exterior of respondent’s car while he was lawfully seized for a traffic violation. Any intrusion on respondent’s privacy expectations does not rise to the level of a constitutionally cognizable infringement.

    This conclusion is entirely consistent with our recent decision that the use of a thermal-imaging device to detect the growth of marijuana in a home constituted an unlawful search. Kyllo v. United States, 533 U.S. 27 (2001). Critical to that decision was the fact that the device was capable of detecting lawful activity—in that case, intimate details in a home, such as “at what hour each night the lady of the house takes her daily sauna and bath.” The legitimate expectation that information about perfectly lawful activity will remain private is categorically distinguishable from respondent’s hopes or expectations concerning the nondetection of contraband in the trunk of his car. A dog sniff conducted during a concededly lawful traffic stop that reveals no information other than the location of a substance that no individual has any right to possess does not violate the Fourth Amendment.

    The judgment of the Illinois Supreme Court is vacated, and the case is remanded for further proceedings not inconsistent with this opinion.

    Justice SOUTER, dissenting.

    I would hold that using the dog for the purposes of determining the presence of marijuana in the car’s trunk was a search unauthorized as an incident of the speeding stop and unjustified on any other ground. I would accordingly affirm the judgment of the Supreme Court of Illinois, and I respectfully dissent.

    At the heart both of Place and the Court’s opinion today is the proposition that sniffs by a trained dog are sui generis because a reaction by the dog in going alert is a response to nothing but the presence of contraband. Hence, the argument goes, because the sniff can only reveal the presence of items devoid of any legal use, the sniff “does not implicate legitimate privacy interests” and is not to be treated as a search.

    The infallible dog, however, is a creature of legal fiction. Although the Supreme Court of Illinois did not get into the sniffing averages of drug dogs, their supposed infallibility is belied by judicial opinions describing well-trained animals sniffing and alerting with less than perfect accuracy, whether owing to errors by their handlers, the limitations of the dogs themselves, or even the pervasive contamination of currency by cocaine. Indeed, a study cited by Illinois in this case for the proposition that dog sniffs are “generally reliable” shows that dogs in artificial testing situations return false positives anywhere from 12.5% to 60% of the time, depending on the length of the search. In practical terms, the evidence is clear that the dog that alerts hundreds of times will be wrong dozens of times.

    Once the dog’s fallibility is recognized, however, that ends the justification claimed in Place for treating the sniff as sui generis under the Fourth Amendment: the sniff alert does not necessarily signal hidden contraband, and opening the container or enclosed space whose emanations the dog has sensed will not necessarily reveal contraband or any other evidence of crime. This is not, of course, to deny that a dog’s reaction may provide reasonable suspicion, or probable cause, to search the container or enclosure; the Fourth Amendment does not demand certainty of success to justify a search for evidence or contraband. The point is simply that the sniff and alert cannot claim the certainty that Place assumed, both in treating the deliberate use of sniffing dogs as sui generis and then taking that characterization as a reason to say they are not searches subject to Fourth Amendment scrutiny. And when that aura of uniqueness disappears, there is no basis in Place’s reasoning, and no good reason otherwise, to ignore the actual function that dog sniffs perform. They are conducted to obtain information about the contents of private spaces beyond anything that human senses could perceive, even when conventionally enhanced. The information is not provided by independent third parties beyond the reach of constitutional limitations, but gathered by the government’s own officers in order to justify searches of the traditional sort, which may or may not reveal evidence of crime but will disclose anything meant to be kept private in the area searched. Thus in practice the government’s use of a trained narcotics dog functions as a limited search to reveal undisclosed facts about private enclosures, to be used to justify a further and complete search of the enclosed area. And given the fallibility of the dog, the sniff is the first step in a process that may disclose “intimate details” without revealing contraband, just as a thermal-imaging device might do, as described in Kyllo v. United States.

    It makes sense, then, to treat a sniff as the search that it amounts to in practice, and to rely on the body of our Fourth Amendment cases, including Kyllo, in deciding whether such a search is reasonable. As a general proposition, using a dog to sniff for drugs is subject to the rule that the object of enforcing criminal laws does not, without more, justify suspicionless Fourth Amendment intrusions. Since the police claim to have had no particular suspicion that Caballes was violating any drug law, this sniff search must stand or fall on its being ancillary to the traffic stop that led up to it.

    For the sake of providing a workable framework to analyze cases on facts like these, which are certain to come along, I would treat the dog sniff as the familiar search it is in fact, subject to scrutiny under the Fourth Amendment.1

    Justice GINSBURG, with whom Justice SOUTER joins, dissenting.

    The Court has never removed police action from Fourth Amendment control on the ground that the action is well calculated to apprehend the guilty. Under today’s decision, every traffic stop could become an occasion to call in the dogs, to the distress and embarrassment of the law-abiding population.

    The Illinois Supreme Court, it seems to me, correctly apprehended the danger in allowing the police to search for contraband despite the absence of cause to suspect its presence. Today’s decision, in contrast, clears the way for suspicionless, dog-accompanied drug sweeps of parked cars along sidewalks and in parking lots. Nor would motorists have constitutional grounds for complaint should police with dogs, stationed at long traffic lights, circle cars waiting for the red signal to turn green.

    Today’s decision also undermines this Court’s situation-sensitive balancing of Fourth Amendment interests in other contexts. For example, in Bond v. United States, 529 U.S. 334 (2000), the Court held that a bus passenger had an expectation of privacy in a bag placed in an overhead bin and that a police officer’s physical manipulation of the bag constituted an illegal search. If canine drug sniffs are entirely exempt from Fourth Amendment inspection, a sniff could substitute for an officer’s request to a bus passenger for permission to search his bag, with this significant difference: The passenger would not have the option to say “No.”

    The dog sniff in this case, it bears emphasis, was for drug detection only. A dog sniff for explosives, involving security interests not presented here, would be an entirely different matter.

    For the reasons stated, I would hold that the police violated Caballes’ Fourth Amendment rights when, without cause to suspect wrongdoing, they conducted a dog sniff of his vehicle. I would therefore affirm the judgment of the Illinois Supreme Court.

    * * *

    The previous two cases analyzed the use of dogs under the “reasonable expectation of privacy” test derived from Katz. In the next case, which came one year after United States v. Jones, the Court considered the use of dogs under the law of trespass.

    Supreme Court of the United States

    Florida v. Joelis Jardines

    Decided March 26, 2013 – 569 U.S. 1

    Justice SCALIA delivered the opinion of the Court.

    We consider whether using a drug-sniffing dog on a homeowner’s porch to investigate the contents of the home is a “search” within the meaning of the Fourth Amendment.


    In 2006, Detective William Pedraja of the Miami-Dade Police Department received an unverified tip that marijuana was being grown in the home of respondent Joelis Jardines. One month later, the Department and the Drug Enforcement Administration sent a joint surveillance team to Jardines’ home. Detective Pedraja was part of that team. He watched the home for fifteen minutes and saw no vehicles in the driveway or activity around the home, and could not see inside because the blinds were drawn. Detective Pedraja then approached Jardines’ home accompanied by Detective Douglas Bartelt, a trained canine handler who had just arrived at the scene with his drug-sniffing dog. The dog was trained to detect the scent of marijuana, cocaine, heroin, and several other drugs, indicating the presence of any of these substances through particular behavioral changes recognizable by his handler.

    Detective Bartelt had the dog on a six-foot leash, owing in part to the dog’s “wild” nature and tendency to dart around erratically while searching. As the dog approached Jardines’ front porch, he apparently sensed one of the odors he had been trained to detect, and began energetically exploring the area for the strongest point source of that odor. As Detective Bartelt explained, the dog “began tracking that airborne odor by … tracking back and forth,” engaging in what is called “bracketing,” “back and forth, back and forth.” Detective Bartelt gave the dog “the full six feet of the leash plus whatever safe distance [he could] give him” to do this—he testified that he needed to give the dog “as much distance as I can.” And Detective Pedraja stood back while this was occurring, so that he would not “get knocked over” when the dog was “spinning around trying to find” the source.

    After sniffing the base of the front door, the dog sat, which is the trained behavior upon discovering the odor’s strongest point. Detective Bartelt then pulled the dog away from the door and returned to his vehicle. He left the scene after informing Detective Pedraja that there had been a positive alert for narcotics.

    On the basis of what he had learned at the home, Detective Pedraja applied for and received a warrant to search the residence. When the warrant was executed later that day, Jardines attempted to flee and was arrested; the search revealed marijuana plants, and he was charged with trafficking in cannabis.

    At trial, Jardines moved to suppress the marijuana plants on the ground that the canine investigation was an unreasonable search. The trial court granted the motion, and the Florida Third District Court of Appeal reversed. On a petition for discretionary review, the Florida Supreme Court quashed the decision of the Third District Court of Appeal and approved the trial court’s decision to suppress, holding (as relevant here) that the use of the trained narcotics dog to investigate Jardines’ home was a Fourth Amendment search unsupported by probable cause, rendering invalid the warrant based upon information gathered in that search.

    We granted certiorari, limited to the question of whether the officers’ behavior was a search within the meaning of the Fourth Amendment.


    [W]hen it comes to the Fourth Amendment, the home is first among equals. At the Amendment’s “very core” stands “the right of a man to retreat into his own home and there be free from unreasonable governmental intrusion.” This right would be of little practical value if the State’s agents could stand in a home’s porch or side garden and trawl for evidence with impunity; the right to retreat would be significantly diminished if the police could enter a man’s property to observe his repose from just outside the front window.

    We therefore regard the area “immediately surrounding and associated with the home”—what our cases call the curtilage—as “part of the home itself for Fourth Amendment purposes.” Here there is no doubt that the officers entered it: The front porch is the classic exemplar of an area adjacent to the home and “to which the activity of home life extends.”

    Since the officers’ investigation took place in a constitutionally protected area, we turn to the question of whether it was accomplished through an unlicensed physical intrusion. While law enforcement officers need not “shield their eyes” when passing by the home “on public thoroughfares,” an officer’s leave to gather information is sharply circumscribed when he steps off those thoroughfares and enters the Fourth Amendment’s protected areas. As it is undisputed that the detectives had all four of their feet and all four of their companion’s firmly planted on the constitutionally protected extension of Jardines’ home, the only question is whether he had given his leave (even implicitly) for them to do so. He had not.

    We have recognized that “the knocker on the front door is treated as an invitation or license to attempt an entry, justifying ingress to the home by solicitors, hawkers and peddlers of all kinds.” This implicit license typically permits the visitor to approach the home by the front path, knock promptly, wait briefly to be received, and then (absent invitation to linger longer) leave. Complying with the terms of that traditional invitation does not require fine-grained legal knowledge; it is generally managed without incident by the Nation’s Girl Scouts and trick-or-treaters. Thus, a police officer not armed with a warrant may approach a home and knock, precisely because that is “no more than any private citizen might do.”

    But introducing a trained police dog to explore the area around the home in hopes of discovering incriminating evidence is something else. There is no customary invitation to do that. An invitation to engage in canine forensic investigation assuredly does not inhere in the very act of hanging a knocker. To find a visitor knocking on the door is routine (even if sometimes unwelcome); to spot that same visitor exploring the front path with a metal detector, or marching his bloodhound into the garden before saying hello and asking permission, would inspire most of us to—well, call the police. The scope of a license—express or implied—is limited not only to a particular area but also to a specific purpose. Consent at a traffic stop to an officer’s checking out an anonymous tip that there is a body in the trunk does not permit the officer to rummage through the trunk for narcotics. Here, the background social norms that invite a visitor to the front door do not invite him there to conduct a search.

    The government’s use of trained police dogs to investigate the home and its immediate surroundings is a “search” within the meaning of the Fourth Amendment. The judgment of the Supreme Court of Florida is therefore affirmed.

    Notes, Comments, and Questions

    Does the outcome change if the dog is sniffing the door of an apartment instead of a home? Consider a police officer who is investigating an individual for methamphetamine production. The individual lives on the third floor of an apartment building. The police officer leads a dog to the third-floor hallway; the dog sniffs several doors in the hallway without alerting. While sniffing the suspect’s door, the dog alerts to the presence of drugs. Search or no search? Why or why not? See State v. Edstrom, 916 N.W.2d 512, 515 (Minn. 2018), cert. denied, 139 S. Ct. 1262 (2019).

    This chapter’s final dog case differs somewhat from the previous cases. Instead of considering what activity by dogs counts as a “search,” the Court considers how well trained a police dog must be for its “alert” to count toward the probable cause needed to justify a vehicle search. The more reliable a dog is, the more reasonable it is for police to search an area to which the dog has alerted. The less reliable the dog, the less reasonable police reliance becomes.

    Supreme Court of the United States

    Florida v. Clayton Harris

    Decided Feb. 19, 2013 – 568 U.S. 237

    Justice KAGAN delivered the opinion of the [unanimous] Court.

    In this case, we consider how a court should determine if the “alert” of a drug-detection dog during a traffic stop provides probable cause to search a vehicle. The Florida Supreme Court held that the State must in every case present an exhaustive set of records, including a log of the dog’s performance in the field, to establish the dog’s reliability. We think that demand inconsistent with the “flexible, common-sense standard” of probable cause.


    William Wheetley is a K–9 Officer in the Liberty County, Florida Sheriff’s Office. On June 24, 2006, he was on a routine patrol with Aldo, a German shepherd trained to detect certain narcotics (methamphetamine, marijuana, cocaine, heroin, and ecstasy). Wheetley pulled over respondent Clayton Harris’s truck because it had an expired license plate. On approaching the driver’s-side door, Wheetley saw that Harris was “visibly nervous,” unable to sit still, shaking, and breathing rapidly. Wheetley also noticed an open can of beer in the truck’s cup holder. Wheetley asked Harris for consent to search the truck, but Harris refused. At that point, Wheetley retrieved Aldo from the patrol car and walked him around Harris’s truck for a “free air sniff.” Aldo alerted at the driver’s-side door handle—signaling, through a distinctive set of behaviors, that he smelled drugs there.

    Wheetley concluded, based principally on Aldo’s alert, that he had probable cause to search the truck. His search did not turn up any of the drugs Aldo was trained to detect. But it did reveal 200 loose pseudoephedrine pills, 8,000 matches, a bottle of hydrochloric acid, two containers of antifreeze, and a coffee filter full of iodine crystals—all ingredients for making methamphetamine. Wheetley accordingly arrested Harris, who admitted after proper Miranda warnings that he routinely “cooked” methamphetamine at his house and could not go “more than a few days without using” it. The State charged Harris with possessing pseudoephedrine for use in manufacturing methamphetamine.

    While out on bail, Harris had another run-in with Wheetley and Aldo. This time, Wheetley pulled Harris over for a broken brake light. Aldo again sniffed the truck’s exterior, and again alerted at the driver’s-side door handle. Wheetley once more searched the truck, but on this occasion discovered nothing of interest.

    Harris moved to suppress the evidence found in his truck on the ground that Aldo’s alert had not given Wheetley probable cause for a search. At the hearing on that motion, Wheetley testified about both his and Aldo’s training in drug detection. Wheetley (and a different dog) completed a 160-hour course in narcotics detection offered by the Dothan, Alabama Police Department, while Aldo (and a different handler) completed a similar, 120-hour course given by the Apopka, Florida Police Department. That same year, Aldo received a one-year certification from Drug Beat, a private company that specializes in testing and certifying K-9 dogs. Wheetley and Aldo teamed up in 2005 and went through another, 40-hour refresher course in Dothan together. They also did four hours of training exercises each week to maintain their skills. Wheetley would hide drugs in certain vehicles or buildings while leaving others “blank” to determine whether Aldo alerted at the right places. According to Wheetley, Aldo’s performance in those exercises was “really good.” The State introduced “Monthly Canine Detection Training Logs” consistent with that testimony: They showed that Aldo always found hidden drugs and that he performed “satisfactorily” (the higher of two possible assessments) on each day of training.

    On cross-examination, Harris’s attorney chose not to contest the quality of Aldo’s or Wheetley’s training. She focused instead on Aldo’s certification and his performance in the field, particularly the two stops of Harris’s truck. Wheetley conceded that the certification (which, he noted, Florida law did not require) had expired the year before he pulled Harris over. Wheetley also acknowledged that he did not keep complete records of Aldo’s performance in traffic stops or other field work; instead, he maintained records only of alerts resulting in arrests. But Wheetley defended Aldo’s two alerts to Harris’s seemingly narcotics-free truck: According to Wheetley, Harris probably transferred the odor of methamphetamine to the door handle, and Aldo responded to that “residual odor.”

    The trial court concluded that Wheetley had probable cause to search Harris’s truck and so denied the motion to suppress. Harris then entered a no-contest plea while reserving the right to appeal the trial court’s ruling. An intermediate state court summarily affirmed.

    The Florida Supreme Court reversed, holding that Wheetley lacked probable cause to search Harris’s vehicle under the Fourth Amendment. “[W]hen a dog alerts,” the court wrote, “the fact that the dog has been trained and certified is simply not enough to establish probable cause.” To demonstrate a dog’s reliability, the State needed to produce a wider array of evidence:

    “[T]he State must present … the dog’s training and certification records, an explanation of the meaning of the particular training and certification, field performance records (including any unverified alerts), and evidence concerning the experience and training of the officer handling the dog, as well as any other objective evidence known to the officer about the dog’s reliability.”

    The court particularly stressed the need for “evidence of the dog’s performance history,” including records showing “how often the dog has alerted in the field without illegal contraband having been found.” That data, the court stated, could help to expose such problems as a handler’s tendency (conscious or not) to “cue [a] dog to alert” and “a dog’s inability to distinguish between residual odors and actual drugs.” Accordingly, an officer like Wheetley who did not keep full records of his dog’s field performance could never have the requisite cause to think “that the dog is a reliable indicator of drugs.”

    We granted certiorari and now reverse.


    A police officer has probable cause to conduct a search when “the facts available to [him] would ‘warrant a [person] of reasonable caution in the belief’” that contraband or evidence of a crime is present. The test for probable cause is not reducible to “precise definition or quantification.” “Finely tuned standards such as proof beyond a reasonable doubt or by a preponderance of the evidence … have no place in the [probable-cause] decision.” All we have required is the kind of “fair probability” on which “reasonable and prudent [people,] not legal technicians, act.”

    In evaluating whether the State has met this practical and common-sensical standard, we have consistently looked to the totality of the circumstances. We have rejected rigid rules, bright-line tests, and mechanistic inquiries in favor of a more flexible, all-things-considered approach. Probable cause, is “a fluid concept—turning on the assessment of probabilities in particular factual contexts—not readily, or even usefully, reduced to a neat set of legal rules.”

    The Florida Supreme Court flouted this established approach to determining probable cause. To assess the reliability of a drug-detection dog, the court created a strict evidentiary checklist, whose every item the State must tick off. Most prominently, an alert cannot establish probable cause under the Florida court’s decision unless the State introduces comprehensive documentation of the dog’s prior “hits” and “misses” in the field. (One wonders how the court would apply its test to a rookie dog.) No matter how much other proof the State offers of the dog’s reliability, the absent field performance records will preclude a finding of probable cause. That is the antithesis of a totality-of-the-circumstances analysis. [A] finding of a drug-detection dog’s reliability cannot depend on the State’s satisfaction of multiple, independent evidentiary requirements. No more for dogs than for human informants is such an inflexible checklist the way to prove reliability, and thus establish probable cause.

    Making matters worse, the decision below treats records of a dog’s field performance as the gold standard in evidence, when in most cases they have relatively limited import. Errors may abound in such records. If a dog on patrol fails to alert to a car containing drugs, the mistake usually will go undetected because the officer will not initiate a search. Field data thus may not capture a dog’s false negatives. Conversely (and more relevant here), if the dog alerts to a car in which the officer finds no narcotics, the dog may not have made a mistake at all. The dog may have detected substances that were too well hidden or present in quantities too small for the officer to locate. Or the dog may have smelled the residual odor of drugs previously in the vehicle or on the driver’s person. Field data thus may markedly overstate a dog’s real false positives. By contrast, those inaccuracies—in either direction—do not taint records of a dog’s performance in standard training and certification settings. There, the designers of an assessment know where drugs are hidden and where they are not—and so where a dog should alert and where he should not. The better measure of a dog’s reliability thus comes away from the field, in controlled testing environments.

    For that reason, evidence of a dog’s satisfactory performance in a certification or training program can itself provide sufficient reason to trust his alert. If a bona fide organization has certified a dog after testing his reliability in a controlled setting, a court can presume (subject to any conflicting evidence offered) that the dog’s alert provides probable cause to search. The same is true, even in the absence of formal certification, if the dog has recently and successfully completed a training program that evaluated his proficiency in locating drugs. After all, law enforcement units have their own strong incentive to use effective training and certification programs, because only accurate drug-detection dogs enable officers to locate contraband without incurring unnecessary risks or wasting limited time and resources.

    A defendant, however, must have an opportunity to challenge such evidence of a dog’s reliability, whether by cross-examining the testifying officer or by introducing his own fact or expert witnesses. The defendant, for example, may contest the adequacy of a certification or training program, perhaps asserting that its standards are too lax or its methods faulty. So too, the defendant may examine how the dog (or handler) performed in the assessments made in those settings. Indeed, evidence of the dog’s (or handler’s) history in the field, although susceptible to the kind of misinterpretation we have discussed, may sometimes be relevant, as the Solicitor General acknowledged at oral argument. And even assuming a dog is generally reliable, circumstances surrounding a particular alert may undermine the case for probable cause—if, say, the officer cued the dog (consciously or not), or if the team was working under unfamiliar conditions.

    In short, a probable-cause hearing focusing on a dog’s alert should proceed much like any other. The court should allow the parties to make their best case, consistent with the usual rules of criminal procedure. And the court should then evaluate the proffered evidence to decide what all the circumstances demonstrate. If the State has produced proof from controlled settings that a dog performs reliably in detecting drugs, and the defendant has not contested that showing, then the court should find probable cause. If, in contrast, the defendant has challenged the State’s case (by disputing the reliability of the dog overall or of a particular alert), then the court should weigh the competing evidence. In all events, the court should not prescribe, as the Florida Supreme Court did, an inflexible set of evidentiary requirements. The question—similar to every inquiry into probable cause—is whether all the facts surrounding a dog’s alert, viewed through the lens of common sense, would make a reasonably prudent person think that a search would reveal contraband or evidence of a crime. A sniff is up to snuff when it meets that test.


    And here, Aldo’s did. The record in this case amply supported the trial court’s determination that Aldo’s alert gave Wheetley probable cause to search Harris’s truck.

    Because training records established Aldo’s reliability in detecting drugs and Harris failed to undermine that showing, we agree with the trial court that Wheetley had probable cause to search Harris’s truck. We accordingly reverse the judgment of the Florida Supreme Court.

    Notes, Comments, and Questions

    In Harris, the Court held that Aldo (the police dog) was reliable enough that his “alert” provided sufficient evidence of crime that officers had “probable cause” to search a vehicle. The term “probable cause” appears in the text of the Fourth Amendment, and its definition is essential to understanding when police may obtain warrants, when they may search cars, and many other important questions. We examine the concept of probable cause in some detail in the next chapter.

    When studying dog alerts, students should remember that lawyers have many opportunities to object to what they consider unreliable dog-alert evidence. In Harris, defense counsel’s goal was to exclude the evidence police found while searching a truck. A different Florida case, State v. Merrit Alonzo Sims (Fla. 2007), the Supreme Court of Florida considered a lawyer’s failure to object to dog-alert evidence for a different reason. In that case, a police dog handler testified at a murder trial “that his dog alerted him to the presence of narcotics in the passenger side of the car that Sims was driving.” Sims had admitted to killing a police officer with the officer’s police pistol; the issue at trial was whether the killing was murder or self-defense. Sims claimed the officer “had choked him, used racial epithets, and repeatedly threatened to kill him.”

    The state, by contrast, argued that because Sims had drugs in his car, he had a motive to kill the officer to avoid being returned to prison (drug possession was against the terms of Sims’s parole). One difficulty for the state was that drugs were not found when the car was searched sometime after the killing. The state argued “that the dog would alert to the scent of narcotics after the drugs had been removed, and … used this to develop its theory of Sims’ motive.” After Sims had spent more than a decade on death row, the Florida Supreme Court considered whether his trial counsel provided ineffective assistance of counsel by failing to object to the dog-alert evidence (that is, to the testimony by the dog handler). The court held that the evidence was so unreliable that it would have been excluded had counsel properly objected, that counsel had no good justification for that failure, and that Sims’s conviction must be set aside. Sims would later plead guilty, taking a deal that included a sentence of 25 years’ imprisonment—and no death penalty.

    This is but one example of a reason one might object to unreliable evidence (of all kinds). Further, even if a lawyer cannot win the exclusion of unreliable evidence, she can still argue to the jury that the evidence is lousy and should be disregarded. When studying legal doctrines related to the exclusion of evidence, students should not forget that the most common ways to attack “bad” evidence involve a combination of argument and contrary evidence.

    Search Review

    Fourth Amendment: What Is a Search?

    Before moving to the next chapter, students may wish to review the definition of “search” by considering these examples. Instructions: Write “is,” “is not,” or “may be” in each blank. If your answer is “may be,” jot down in the margin why you are unsure. Each problem is independent of all other ones.

    1) If a police officer uses a car to follow a suspect who is driving from home to work, that _________________ a search.

    2) If a police officer flies a helicopter fifty feet above the ground and uses binoculars to look into a house window, that _________________ a search.

    3) If a police officer rifles through a suspect’s paper recycling before the sanitation department collects it (and removes an itemized credit card bill), that _________________ a search.

    4) If a police officer borrows a rare super-sensitive microphone from the CIA and points it at a living room window from across the street, thereby capturing the window vibrations and listening to the conversations of people inside, that _________________ a search.

    5) If a police department deploys officers in shifts 24/7 to watch a house, writing down the description of everyone who comes and goes, that _________________ a search.

    6) If a police officer chases a robbery suspect from the scene of a bank robbery, and the officer follows the sprinting subject into a nearby house, that _________________ a search.