A federal judge in Kentucky this week upheld a lower court's decision to throw out crucial evidence in a drug case because the evidence was gathered with the help of a GPS tracking device that was installed without a warrant on the suspect's vehicle.

In a 19-page ruling Tuesday, Judge Amul Thapar of the U.S. District Court for the Eastern District of Kentucky wrote that Robert Lee's constitutional rights were violated when drug enforcement agents illegally tracked his car and then seized 150 pounds of marijuana from it.

Thapar granted Lee's motion to suppress the evidence, noting that it had been obtained purely as the result of a fishing expedition. "In this case, the DEA agents had their fishing poles out to catch Lee.," Thapar wrote. "Admittedly, the agents did not intend to break the law. But they installed a GPS device on Lee's car without a warrant 'in the hope that something might turn up,'" he said.

The Supreme Court earlier this year ruled that law enforcement officials needed search warrants before they could track suspects using GPS devices. The ruling involved someone who was convicted on drug distribution charges based on evidence gathered by police who tracked his movements using an illegally installed GPS tracker.

Despite the Supreme Court ruling, several lower courts have in recent months permitted evidence gathered through warrantless GPS tracking to be used to prosecute individuals.

In the Ninth Circuit, for instance, three district courts have so far this year applied a 'good-faith exemption' to permit evidence from warrantless GPS tracking, Thapar noted. One district court in the Eight Circuit has also done the same.

In each of these cases, the courts held that the warrantless tracking was exempt from the Supreme Court ruling because it was conducted before the ruling and because appeals courts in both the Eighth Circuit and the Ninth Circuit have previously permitted warrantless GPS tracking.

The same reasoning cannot be applied to Lee's case because there has been no similar precedent established by the Sixth Circuit Court of Appeal. "When carrying out searches, federal officers need only know the binding decisions of the Supreme Court and their circuit's court of appeals," Thapar noted.

In the absence of a precedent in the Sixth Circuit, the officers acted illegally when they attached the GPS tracker to Lee's car without a warrant, he wrote.

"The DEA agents in this case did not rely on any binding appellate precedent," Thapar wrote. "Neither the Sixth Circuit nor the Supreme Court had spoken on the issue of GPS surveillance when the agents placed the tracking device on Lee's car."

Lee was arrested in September 2011 on his way back to London, Ky. after a trip to Chicago. Lee had just completed a period of supervised release after a 42-month stint in prison on marijuana distribution and firearms possession charges.

While Lee was meeting with his probation officer, an agent of the Drug Enforcement Administration (DEA) secretly attached a real-time GPS tracker to the underside of his vehicle without obtaining court approval first.

A few days later, DEA agents noticed him travelling to Chicago and suspected him of buying drugs there based on information they had previously obtained from an informant. The agents instructed a state trooper to find some pretext to stop Lee's vehicle telling him the car probably contained marijuana. The marijuana was discovered shortly after the trooper stopped Lee for driving without a seatbelt.

During the entire process, DEA agents tracked Lee's movements using the GPS tracker and even instructed the state trooper on where to intercept Lee's car.

Jaikumar Vijayan covers data security and privacy issues, financial services security and e-voting for Computerworld. Follow Jaikumar on Twitter at @jaivijayan or subscribe to Jaikumar's RSS feed . His e-mail address is jvijayan@computerworld.com.

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