The case centers on the increasingly detailed information that companies keep on texts and calls to and from a cellphone, as well as the relatively precise location that can be gleaned from that information.

The question is whether cellphone users can expect that data to remain private and subject to the Fourth Amendment’s protection from unreasonable searches and seizures.

In this case, investigators did not need a warrant to obtain more than five months of cellphone location information on Michigan robbery suspect Timothy Carpenter. Instead, they used a federal law ( PL 99-508 ) that governs digital business records. To get a warrant for the information, investigators would have had to establish probable cause, a more difficult standard to meet.

The American Civil Liberties Union, which represents Carpenter, said in a statement that the records in this case demonstrate how location data “can reveal extraordinarily private details about people’s lives, from where they sleep to where they pray.”

“For example, the location data showed that in the early afternoon on a number of Sundays, Carpenter made or received calls from the cell tower sectors nearest to his church,” the ACLU statement said. “His cell phone records do not routinely show him in that area on other days of the week, implying that he was worshipping at those times.”

The U.S. Court of Appeals for the 6th Circuit in April sided with the investigators, and found cellphone users have no reasonable expectation of privacy in cellphone location records held by mobile service providers. Investigators sought business records about routing information of communications maintained by the service provider, the appeals court decided, and people knowingly expose that information to the mobile company.

But one 6th Circuit judge wrote in that decision that “the sheer quantity of sensitive information procured without a warrant” raised Fourth Amendment concerns.

The government had urged the Supreme Court not to review the case, arguing that the procedure used by investigators in the case is constitutional.

Police seek these kinds of cellphone location records tens of thousands of times each year, the ACLU said.

“Because cellphone location records can reveal countless private details of our lives, police should only be able to access them by getting a warrant based on probable cause,” said Nathan Freed Wessler, a staff attorney with the ACLU Speech, Privacy, and Technology Project.

The case is Timothy Carpenter v. United States, Docket No. 16-402.