An appellate court has finally supplied an answer to an open question left dangling by the Supreme Court in 2012: Do law enforcement agencies need a probable-cause warrant to affix a GPS tracker to a target’s vehicle?

The Third Circuit Court of Appeals gave a resounding yes to that question today in a 2 to 1 decision.

“Today’s decision is 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,” said ACLU attorney Catherine Crump in a statement. “These protections are important because where people go reveals a great deal about them, from who their friends are, where they visit the doctor and where they choose to worship.”

It’s the first appeals court ruling in the wake of United States v. Jones, a Supreme Court case involving a convicted drug dealer. In that case, the Supreme Court justices ruled in January 2012 that law enforcement’s installation of a GPS device on a target’s vehicle constituted a search under the Fourth Amendment. The justices declined to rule at the time, however, on whether such a search was unreasonable and therefore required a warrant.

A number of court cases in the wake of Jones have grappled with the question of GPS trackers, but all of these cases have involved the use of GPS trackers prior to the Supreme Court decision. In these cases, the courts had to decide whether evidence obtained through the use of GPS trackers was still admissible in light of the Supreme Court decision. Several courts around the country have ruled that the evidence gathered prior to the Supreme Court ruling can be submitted in court because investigators were acting in good faith at the time, relying on what were then binding rulings in several U.S. Circuit Courts of Appeal that authorized the use of warrantless GPS trackers for surveillance.

Circuit courts in the 7th (covering Illinois, Wisconsin and Indiana), 8th (covering Arkansas, Iowa, Minnesota, Missouri, Nebraska, North Dakota and South Dakota) and 9th (covering Alaska, Arizona, California, Guam, Hawaii, Idaho, the Mariana Islands, Montana, Nevada, Oregon and Washington) all ruled prior to the Supreme Court case that warrantless GPS tracking was legal. Although the Third Circuit Court, where the current case was heard, did not have such a ruling in place prior to the Supreme Court decision, the government argued that investigators in this most recent case were operating under good faith based on prior rulings in these other districts.

The appellate judges rejected this argument, however, and ruled that the “good faith” exception did not apply.

The appellate case, U.S. vs. Harry Katzin, Michael Katzin and Mark Louis Katzin, Sr., involves a string of pharmacy burglaries committed in Delaware, Maryland and New Jersey in 2009 and 2010.

The crimes occurred mostly at Rite-Aid pharmacies, where the thieves cut external phone lines to disable the alarm systems. Harry Katzin was an electrician who, along with his two brothers, became targets of the investigation. They had criminal histories and were seen by police on a few occasions lurking outside Rite Aids prior to the lines being cut at the stores.

The police knew that Harry Katzin regularly parked his van on a particular street in Philadelphia so one early morning in December, after consulting with the United States Attorney’s office, but without obtaining a warrant, the FBI affixed a “slap-on” GPS tracker to the exterior of Katzin’s van. Using the device, investigators were able to map a trail directly connecting Katzin’s movements to a Rite Aid store. When they discovered that the store had been burglarized shortly afterward, police pulled over Katzin’s van and discovered all three brothers inside as well as merchandise from the store inside the van, including pill bottles and Rite Aid storage bins.

The brothers filed a motion to suppress the evidence. The government argued that a warrant was not required for the tracker and that the search of the car was based on reasonable suspicion. The government also argued that if officers were required to obtain a warrant and have probable cause prior to executing a GPS search, “officers could not use GPS devices to gather information to establish probable cause, which is often the most productive use of such devices.”

The justices said the government’s statement “wags the dog rather vigorously,” noting that the primary reason for a search cannot be to generate evidence for law enforcement purposes. They also noted that “Generally speaking, a warrantless search is not rendered reasonable merely because probable cause existed that would have justified the issuance of a warrant.”

The justices also rejected the government’s argument that obtaining a warrant would impede the ability of law enforcement to investigate crimes.

“Consequently, we hold that — absent some highly specific circumstances not present in this case — the police cannot justify a warrantless GPS search with reasonable suspicion alone,” the justices wrote.