This past week, the United States Supreme Court heard oral argument in Carpenter v. United States, a case that goes to the heart of the government’s power to track private individual behavior without a warrant. Timothy Carpenter and his henchmen had engaged in a series of armed robberies in Michigan and Ohio that led to his conviction for up to 116 years in prison. In his opinion, Judge Raymond Kethledge of the Sixth Circuit correctly observed that the Fourth Amendment’s protection against unreasonable searches and seizures “has long recognized a distinction between the content of a communication and the information necessary to convey it. Content, per this distinction, is protected under the Fourth Amendment, but routing information is not.”

Accordingly, Kethledge let the government introduce into evidence the business records from the defendants’ wireless carriers that placed the defendants at or near the scene of several violent robberies. Alone, that evidence could never support a conviction for armed robbery, but its value lies in contravening the defendants’ alibi that they were elsewhere at the time.

Kethledge’s analysis received a rocky reception at the Supreme Court, as both liberal and conservative’s fretted about the invasions of privacy from such extended surveillance. A puzzled Justice Stephen Breyer professed astonishment by observing: “This is an open box. We know not where to go.” In contrast, Justice Elena Kagan had a more pointed objection. She noted that in United States v. Jones (2012), the Court reversed a conviction for narcotics trafficking because the government had conducted an illegal search by attaching a GPS tracking device to the defendant’s wife’s car without a warrant. According to the majority in that case, attaching the GPS amounted to a common law trespass, which Justice Antonin Scalia held justified suppressing that evidence. To Kagan, the government had to explain how using phone company records differed from using the GPS system, when both give exhaustive and accurate information about the defendant’s whereabouts.

In claiming that Jones governs Carpenter, she sought to update the Fourth Amendment in an age of digital searches. Her underlying claim was that the government’s action violated the defendant’s “reasonable expectation of privacy,” which was first announced in the key 1967 Supreme Court decision of Katz v. United States. Katz held that evidence the government had collected by attaching “an electronic listening and recording device to the outside of the public telephone booth from which [the defendant] had placed his calls” was inadmissible. Even without the trespass, the collection of information from private conversations without a warrant was held illegal.

After Katz, the challenge has been to define a reasonable expectation of privacy. On this point, Kagan rightly notes the equivalence between trespassory and nontrespassory searches, from which she then draws exactly the wrong conclusion. It’s Jones that is wrong, not Carpenter. Social expectations matter, but the trespass does not. The correct approach in both cases is to push the Kethledge line by allowing the government to track a defendant with both GPS and phone data, so long as it can examine the content of any seized information only after it establishes the probable cause needed for a warrant.

Let’s start with the simple observation that the government has long been allowed to use visual surveillance to track the whereabouts of a person, without suspicion and without the probable cause needed to obtain a search warrant. Since the Supreme Court’s critical 1968 decision in Terry v. Ohio, that suspicion justifies a police officer stopping and frisking a potential target, and the information acquired or any weapon seized may be either introduced into evidence or serve as the basis for a warrant to conduct a more intrusive search.

As Justice Scalia noted in Jones, the government decided to track Jones only after it had developed reasonable suspicion that he was engaged in criminal activity. None of the surveillance tactics used to develop this suspicion required a search warrant. The police used ordinary visual surveillance, which they do every time they watch people scurry to and fro on public streets. They also used a hidden camera located outside of Jones’ nightclub. Finally, they used a “pen register,” which recorded phone calls that he had made to other people. In the 1979 case of Smith v. Maryland, Maryland had, without a warrant, requested a phone company to install a special pen register that allowed it to record all the phone numbers dialed from the home of the accused. At trial in Smith, the defendant sought to suppress the use of those numbers as well as “all fruits derived from” the use of the pen register. The court rejected the request on the ground that the accused did not have a “legitimate expectation of privacy” in this information.

The use of that phrase sounds conclusory, no matter how often it is invoked. Nonetheless, the good logic behind the phrase calls attention to the inevitable tradeoff between individual privacy and law enforcement. To shield information of this sort would give criminals enormous leeway. But using it to track down the movements of criminal suspects does much to curtail illegal behavior, with minimal personal intrusion. From behind a social veil of ignorance, we all benefit more from allowing these limited searches notwithstanding the infringement on privacy they entail. The line between content and routing is clear in practice, and recognizing this difference strikes a sensible balance that makes routing searches reasonable, with or without the trespass.

Nor does that balance change when we move from the pen register to comprehensive data collection. The intrusions today are more frequent but they are no more severe. Indeed, the possibility of abuse is less given that there is no need for any special government request. Most critical, using this information allows for better and more directed searches, which reduce the risk of invading the privacy of innocent third parties. And the social gains from these limited searches can be tremendous, when the apprehension of armed robbers weighs on the other side. Here it is always understood that the government needs to have some suspicion to track this data, so that random searches of these connections should not normally be allowed. In addition, the information so collected can only be used in connection with a particular investigation, but otherwise has to remain outside the reach of law enforcement officials.

Once these two limitations on data collection and use are acknowledged, the decisive answer to Justice Kagan is that Carpenter should be affirmed and Jones overruled. Trespass or not, the GPS device did not allow the government to record any conversations or observe any activities by the car’s occupants. Why then attach a device that lets the government cut down on its surveillance costs on the one hand and increase the accuracy of its search on the other? The new technology allows for better authentication of records in controlling police abuse, issuing warrants, and in trying cases. The cash savings could then allow for more extensive criminal investigations subject to the same safeguards. Note, moreover, the law enforcement disaster if Carpenter goes the way of Jones. It would mean surveillance cameras and pen registers would be off limits for tracking terrorists and violent crimes. In addition, the so-called “fruit of the poisonous tree” derived from these limited searches could be inadmissible as well, even if obtained by a warrant. The minimal privacy intrusion is a small price to pay for the increase in overall safety.

Nothing on this view disturbs the 2014 Supreme Court decision in Riley v. California, which forbade the police from searching the digital information contained on the cell phone of an arrested person without a warrant in the absence of the “exigent circumstances” that sometimes justifies a search incident to an arrest. Instead, the correct procedure allows the government to retain possession of the cell phone without search until it accumulates enough probable cause for a search warrant.

Similarly, admitting the tracking evidence in Carpenter does not fortify the so-called “third-party doctrine” announced in the 1976 Supreme Court decision of United States v. Miller, which held that the Fourth Amendment does not protect confidential information that a defendant turns over to someone else for safekeeping such as checks and other bank records. But these government searches do not just track location. It is a non-sequitur to say that the government can examine any information when private people have voluntarily conveyed it to third parties. All explicit social expectations rightly run the other way. The rule in Riley works well here. The government can take custody of the documents until it can get a warrant. But following Riley in no way requires that the law should encourage people to form dangerous expectations, such as that the government cannot track their movements by the best means available prior to the commission of a crime. What matters in these cases is the types of information the government seeks—location or content.

Katz was right to downplay the absence of a trespass. Jones was wrong to play it up. Judge Kethledge got Carpenter right. The Supreme Court should follow his lead.

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