The Supreme Court ruled, at the end of June, that seizing cell-site location information—data that tracks cell phone users’ movements—constitutes a search under the Fourth Amendment. Speaking for a 5-4 majority in Carpenter v. United States, Chief Justice Roberts addressed questions surrounding law enforcement’s warrantless seizure of over 12,000 cell site location points pinged by the defendant’s phone, which allowed for nearly minute-by-minute tracking of his past movements. At the time, police needed only to prove that the data was reasonably relevant to their investigations.  In its opinion the Court will now require that a warrant be obtained with a showing of probable cause – a higher burden of proof than previously required –, and an individualized suspicion that the data’s owner committed a crime in order to access cell-site records.

The case rests at the intersection of two doctrines, both reliant on the limitation of Fourth Amendment protections to areas with a “reasonable expectation of privacy.”

First, people have no reasonable expectation of privacy in their public movements. This is to say that, if an officer observes someone walking down the street, that observation alone does not constitute a search. But, while Carpenter’s movements—as indicated by his cell location data—were public, and could have been observed by anyone who cared to look, the shear ease with which law enforcement could observe him posed problems for the Court. While logistical and budgetary constraints necessarily limit government observation by sight, these constraints don not exist with the collection of cell site data.

Second, a person loses their expectation of privacy once they voluntarily turn over information to a third party. For example, in United States v. Miller the Court did not require a warrant to access the defendant’s bank records, in part because the records were turned over to—and thus belonged to—the bank, not the defendant. However as the Court reasoned, Carpenter’s location data was more expansive than banking documents, allowing law enforcement to create an “all-encompassing record of [his] whereabouts” over the course of four months.  Also cell site data is not “shared” in the same way as bank checks or deposits, which the owner affirmatively hands over to others, instead being shared passively and by way of the phone’s very operation.

Beyond these doctrines’ technical failings, the Court was concerned with the grave intrusion into a person’s privacy that location data affords law enforcement. Said Chief Justice Roberts:

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.

Furthermore, because cell site data can be retrieved retrospectively, police would have “access to a category of information otherwise unknowable.” And as phones become more and more ubiquitous to daily life—indeed, “a feature of human anatomy”—this data could give officers unlimited tracking capabilities.

We will continue to follow these important constitutional privacy issues in the next session of the US Supreme Court and beyond.

Thanks to Mintz Levin summer associate Aaron Fenton for his valuable contribution to this blog post.