Law enforcement agencies must obtain a warrant to get tracking data from mobile phone companies, according to a decision by a federal appeals court.

The Fourth Circuit Court of Appeals ruled (pdf) Wednesday in a 2-1 decision that just because a third party holds information, it does not mean that it can be made freely available to police.

“People cannot be deemed to have volunteered to forfeit expectations of privacy by simply seeking active participation in society through use of their cell phones,” Senate Circuit Judge Andre Davis wrote for the majority.

The case was one of an armed robber whose whereabouts were traced over a seven-month period via data provided to prosecutors by Sprint.

“It’s great for us going forward,” Nate Wessler, a staff attorney with the American Civil Liberties Union’s Speech, Privacy, and Technology Project, told The Intercept. “It’s a robust recognition of how much private information can be revealed through our cell phone records—doctor’s office visits, AA meetings…in the aggregate, it paints a strong picture of our lives.”

In a concurrence, Judge Stephanie Thacker acknowledged the changing privacy landscape brought on by technological advances. “As the march of technological progress continues to advance upon our zone of privacy, each step forward should be met with considered judgment that errs on the side of protecting privacy and accounts for the practical realities of modern life. At bottom, this decision continues a time-honored American tradition—obtaining a warrant is the rule, not the exception.”

The case is now likely to go to the Supreme Court.

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