The US Supreme Court [official website] ruled [opinion, PDF] 5-4 Friday in Carpenter v. United States [SCOTUSblog materials] that police must generally obtain a warrant in order to obtain cell phone location data.

This case arose from petitioner Timothy Carpenter’s conviction for armed robbery, for which he was sentenced to 116 days in prison. At trial, the prosecution offered evidence of Carpenter’s location during the 127 days surrounding the robberies through cell phone records, which law enforcement obtained without a warrant or consent. Carpenter appealed to the Sixth Circuit and then to the Supreme Court, arguing the officers violated his Fourth Amendment expectation of privacy.

The court found first that accessing historical records that provide a user’s past movements constitutes a search under the Fourth Amendment [text]. The court also found that the government must generally obtain a warrant supported by probable cause before acquiring those records.

Chief Justice John Roberts wrote for the majority:

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 [cell-site location information], 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.

Multiple dissents were filed in the case, including one by Justice Anthony Kennedy, which was joined by Justices Clarence Thomas and Samuel Alito, a separate one by Thomas, one by Alito, joined by Thomas, and one by Justice Neil Gorsuch.