A federal appeals court in Philadelphia ruled Tuesday that the government must obtain a warrant to attach a GPS unit to a car.

The case involved alleged pharmacy burglaries in New Jersey, Pennsylvania, and Maryland: the authorities suspected a trio of brothers and slapped a magnetic GPS unit to one of their vehicles after consulting the U.S. Attorney's office -- but without obtaining a warrant. Using the evidence gathered from the device, the vehicle was tracked to a recently burglarized RiteAid. Police stopped the brothers shortly afterward, and a search allegedly revealed items from the RiteAid. In the resulting case, U.S. v. Katzin, the brothers argued that the evidence obtained as a result of the GPS unit should be inadmissible because the police had not obtained a warrant.

The District Court agreed with the brothers, and the government appealed the case to the Court of Appeals for the Third District. On Tuesday, a three-judge panel upheld the lower court's ruling, finding that the actions of the police were "highly disconcerting" under a physical intrusion theory of the Fourth Amendment. The judges dismissed the government's arguments that the search was legal because the police had probable cause even if they didn't seek a warrant, saying "generally speaking, a warrantless search is not rendered reasonable merely because probable cause existed that would have justified the issuance of a warrant."

That's important because it extends a recent Supreme Court ruling, which found that GPS tracking constituted a search but did not rule on whether it's reasonable to conduct such a search without a warrant. This week's ruling is the first time a federal appeals court has ruled since that landmark decision.

The appeals court also rejected a government argument that a GPS search might qualify for the automobile exception, in which police have greater leeway searching through vehicles. "A GPS search," the court found, "extends the police intrusion well past the time it would normally take officers to enter a target vehicle and locate, extract, or examine the then-existing evidence."

American Civil Liberties Union Staff Attorney Catherine Crump, who had argued before the panel, called the decision "a victory for all Americans because it ensures that the police cannot use powerful tracking technology without court supervision and a good reason to believe it will turn up evidence of wrongdoing," in a statement. A request for comment from the U.S. Attorney's Office which argued the appeal was not returned.