Enlarge / A man walks up the steps of the U.S. Supreme Court on January 31, 2017 in Washington, DC. (credit: Mark Wilson/Getty Images)

Cell phones produce “minute-by-minute account of a person’s locations and movements and associations over a long period regardless of what the person is doing at any given moment,” the ACLU’s Nathan Freed Wessler pointed out in an argument before the Supreme Court. The ACLU is urging the Supreme Court to rule the government can’t access these records without a warrant.

But the government pointed to a 1979 Supreme Court ruling called Smith v. Maryland. In that case, the Supreme Court ruled that the government doesn’t need to get a warrant to obtain a customer’s dialing history because they are merely the business records of the phone company. The government argues that the same principle, known as the third-party doctrine, applies here: data about which cell phone towers a customer’s phone has talked to are merely the cell phone company’s business records, and should be available to the government without a warrant.