When the FBI suspected that Timothy Carpenter was involved in several robberies, it identified his cell phone number and obtained cell-site information from his wireless carriers without a warrant, which could be used to track the movement of his phone, and thus Carpenter himself. Carpenter moved to suppress the information as violating the Fourth Amendment’s requirement for a warrant supported by probable cause. The district court denied the motion, and Carpenter was convicted at trial. The Sixth Circuit affirmed, holding that Carpenter lacked a reasonable expectation of privacy over the information held by the wireless carriers. The Court, in a 5-4 decision authored by Chief Justice Roberts, reversed, and held that the acquisition of cell-site records was a Fourth Amendment search that required a warrant. In the intersection of a person’s expectation of privacy in her physical movements, and not having such an expectation of privacy over information turned over to third parties, the majority found the unique nature of cell-site records, and the huge amount of information they provided about a person’s whereabouts, required that they be treated as a search under the Fourth Amendment. Justice Kennedy, joined by Justices Thomas and Alito, filed a dissent arguing that the majority was wrong to treat cell-site records differently from other forms of information held by third parties, which the Government has a right to access without a warrant. Justice Thomas filed a dissent arguing that the “reasonable expectation of privacy” test used by the majority has no basis in Fourth Amendment law and should be discarded. Justice Alito, joined by Justice Thomas, also filed a dissent arguing that the majority’s decision will destabilize Fourth Amendment law and cause more harm than good. Finally, Justice Gorsuch filed a dissent arguing that the Court should return to the more traditional principle of asking whether a given article belongs to someone, regardless of who holds it, to determine the extent of Fourth Amendment rights. A link to the opinion in Carpenter v. United States is here.