Kathy Thomas knew she was under surveillance. The animal rights and environmental activist had been trailed daily by cops over several months, and had even been stopped on occasion by police and FBI agents.

FBI Vehicle-Tracking Device: The Teardown Video: The Dissection of an FBI Bumper-Beeper How to Check Your Car for a GPS TrackerBut when the surveillance seemed to halt suddenly in mid-2005 after she confronted one of the agents, she thought it was all over. Months went by without a peep from the FBI surveillance teams that had been tracking her in undercover vehicles and helicopters. That's when it occurred to her to check her car.

Rumors had been swirling among activists that the FBI might be using GPS to track them – two activists in Colorado discovered mysterious devices attached to their car bumpers in 2003 – so Thomas (a pseudonym) went out to the vehicle in a frenzy and ran her hands beneath the rear bumper. She was only half-surprised to find a small electronic device and foot-long battery wand secured to her metal fender with industrial-strength magnets.

“I think I must have found it right after they put it on, because there was no grime on it at all," she told Threat Level recently.

How Vehicle Tracking Works

Law enforcement secretly installs the tracking device on a target's car. Some models are hidden in the engine compartment and wired to the car battery. Others are slapped to the undercarriage with industrial-strength magnets.

As the target drives around, the tracking device triangulates its position from three or four GPS satellites, and digitally transmits its coordinates continuously by radio.

The law enforcement agency receives the coordinates and displays the target's location in real time on a computerized map, keeping a record of the target's movement. Illustration: Mitsu Overstreet/Wired.com

The use of GPS tracking devices is poised to become one of the most contentious privacy issues before the Supreme Court, if it agrees to hear an appeal filed by the Obama administration last month. The administration is seeking to overturn a ruling by a lower court that law enforcement officials must obtain a warrant before using a tracker.

The constitutional matter until now has been left to district courts around the country to decide, resulting in a patchwork of conflicting rulings. Meanwhile, a federal lawsuit filed in March by an Arab-American college student named Yasir Afifi alleges that the FBI violated his privacy rights by placing a GPS device on his car without a warrant, and that the bureau targeted him simply because of his ethnic background.

In the midst of this legal controversy, Threat Level decided to take a look inside one of the devices – which are generally custom-made for law enforcement. Working with the teardown artists at iFixit, we examined the device Thomas found on her car nearly six years ago, which you can see in the photos and – video accompanying this story.

When Thomas found the device on her vehicle back in 2005, she ripped it from the underside of her fender, but quickly grew fearful the FBI would raid her house if agents suspected she'd removed it. So she carried it in a duffel bag in her trunk for a week, while she and her boyfriend considered what to do.

When her lawyer called a local U.S. attorney to inquire about the device, the prosecutor acknowledged it belonged to the feds and said they wanted it back. But Thomas refused to hand it over, and the FBI seemed to drop the matter. Her attorney told Threat Level the government "basically abandoned it."

She provided it to Threat Level recently after reading a story about Afifi discovering a tracker on his car. She said she wanted to raise more awareness about how the technology is being used for stealth surveillance.

GPS vehicle trackers, based on technology first used by the military for navigation, have become a popular law-enforcement tool for tracking people. Cruder than other forms of surveillance – they report only where a suspect's car goes, not who is in the car or what occupants do when they arrive at a location – they're nonetheless frequently used for supplementary surveillance. That's because in most jurisdictions, investigators don't need court approval to slap a tracking device on a driver's car, and because the devices provide a stealthier and more cost-effective approach to surveillance than a team of cops trailing a suspect around the clock.

The devices, however, have become one of the most divisive Fourth Amendment issues facing courts around the country. The 9th U.S. Circuit Court of Appeals in California ruled last year that using a GPS tracker was no different than physically trailing a suspect in public, and that such surveillance was not protected by the Fourth Amendment, even if agents placed the device on a suspect's car while it was parked in his driveway.

But Judge Alex Kosinski, in the dissenting opinion, called the use of GPS trackers without a court order "straight out of George Orwell's novel 1984" and said they give government "the power to track the movements of every one of us, every day of our lives."

A federal appeals court in Washington, D.C., agreed with him when it ruled in a different case last year that collecting data from a GPS device planted on the Jeep of drug suspect Antoine Jones amounted to a search, and therefore required a warrant. Prosecutors argued that the device only collected the same information anyone on a public street could glean from following the suspect. But Judge Douglas Ginsburg wrote in his ruling that the persistent, nonstop surveillance afforded by a GPS tracker was much different from physically tracking a suspect on a single trip.

"Unlike one's movements during a single journey, the whole of one's movement over the course of a month is not actually exposed to the public because the likelihood anyone will observe all those movements is effectively nil," he wrote. What's more, the bulk of data gleaned by such a device over time could help deduce a lot about a person, such as whether he associated with political groups, was a heavy drinker or weekly churchgoer, was an unfaithful husband or an outpatient receiving regular medical treatment.

The Obama administration called the ruling "vague and unworkable,” and filed a writ in April asking the U.S. Supreme Court to review the case. A decision on whether the high court will hear the case is pending.

It's not known how many people are tracked with GPS devices every year, but the devices don't always go undetected. An elderly Arab-American in the San Francisco Bay Area reportedly discovered a vehicle tracker on his car in 2009, while he attended a free auto-repair workshop and let the instructor demonstrate an oil change on his vehicle.

Then last year, Yasir Afifi, a 20-year-old Arab-American college student in California discovered a device attached to his car when he took the vehicle into an auto shop for an oil change. After a friend posted photos of it on Reddit.com, and readers identified it as a GPS tracker, the FBI showed up at Afifi's apartment demanding he return the device. He's since filed a lawsuit (.pdf) over the tracking.

Although the Justice Department has said the devices are used by investigators "with great frequency," neither the department nor local law enforcement agencies are required to compile or disclose statistics about their use in the way the Justice Department is required to report annually to Congress on the use of national security letters issued to ISPs and other businesses for customer records.

Kathy Thomas doesn't know if the FBI obtained a warrant to place the tracker on her car. But she said authorities never charged her with any crime. Threat Level could find no federal case filed against her.

Her FBI file, which she obtained under a Freedom of Information Act request, makes it clear the surveillance was part of a nationwide investigation of activists connected to Earth First, the Earth Liberation Front and the Animal Liberation Front – groups the FBI considered “left-wing anarchists" whose members sometimes advocated criminal activity to further their aims.

Thomas, who provided Threat Level with only a handful of the 800 redacted pages she received in her request, says she organized activities with Earth First and participated in animal rights activities, but never belonged to the two other groups. Instead, she was a member of Food Not Bombs.

The FBI reports indicate agents likely turned to the GPS tracking device after it became increasingly difficult to tail her physically.

Thomas had begun engaging in countersurveillance maneuvers, FBI agents claimed in the documents, including speeding, running red lights, making unsafe lane changes and weaving through congested traffic to evade them. A July 2004 report describes how she drove one day into the cul-de-sac where she lived and sped around to confront and photograph cars she believed were tailing her. The report says Thomas was becoming "extremely surveillance-conscious," and that agents “were made [recognized as agents] on two separate occasions.”

Thomas says the surveillance was a daily occurrence for months. Then in April 2005 she confronted an agent who was following her on the freeway. She took an exit ramp and stopped, and when he pulled up behind her, she got out of her car to yell at him, shaking a glass Perrier bottle in her hand. She says the agent laughed at her, and after that the surveillance stopped. Or so she thought.

She found the GPS tracker on her car a few months later.

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