In 2013, 18-year-old Tadrae McKenzie robbed a marijuana dealer for $130 worth of pot at a Taco Bell in Tallahassee, Florida. He and two friends had used BB guns to carry out the crime, which under Florida law constitutes robbery with a deadly weapon. McKenzie braced himself to serve the minimum four years in prison.

But in the end, a state judge offered McKenzie a startlingly lenient plea deal: He was ordered to serve only six months’ probation, after pleading guilty to a second-degree misdemeanor. The remarkable deal was related to evidence McKenzie’s defense team uncovered before the trial: Law enforcement had used a secret surveillance tool often called a Stingray to investigate his case.

Stingrays are devices that behave like fake cellphone towers, tricking phones into believing they’re pinging genuine towers nearby. By using the device, cops can determine a suspect’s precise location, outgoing and incoming calls, and even listen-in on a call or see the content of a text message.

Many people may have been convicted using techniques that violated their rights.

McKenzie’s lawyers suspected cops had used a Stingray because they knew exactly where his house was, and knew he left his home at 6 a.m. the day he was arrested. The cops had obtained a court order from a judge to authorize Verizon to hand over data about the location of Mckenzie’s phone. But cell tower data isn’t precise enough to place a device at a specific house.

The cops also said they used a database that lets law enforcement agencies locate individuals by linking them with their phone numbers. But the phone McKenzie was using was a burner, and not associated with his name. Law enforcement couldn’t adequately explain their extraordinary knowledge of his whereabouts.

The state judge in the case ordered police to show the Stingray and its data to McKenzie’s attorneys. They refused, because of a non-disclosure agreement with the FBI. The state then offered McKenzie, as well as the two other defendants, plea deals designed to make the case go away.

The cops in McKenzie’s case had ultimately failed to successfully carry out a troubling technique called “parallel construction.”

First described in government documents obtained by Reuters in 2013, parallel construction is when law enforcement originally obtains evidence through a secret surveillance program, then tries to seek it out again, via normal procedure. In essence, law enforcement creates a parallel, alternative story for how it found information. That way, it can hide surveillance techniques from public scrutiny and would-be criminals.

A new report released by Human Rights Watch Tuesday, based in part on 95 relevant cases, indicates that law enforcement is using parallel construction regularly, though it’s impossible to calculate exactly how often. It’s extremely difficult for defendants to discern when evidence has been obtained via the practice, according to the report.

“When attorneys try to find out if there’s some kind of undisclosed method that’s been used, the prosecution will basically stonewall and try not to provide a definitive yes or no answer,” says Sarah St. Vincent, the author of the report and a national security and surveillance researcher at Human Rights Watch.

In investigation reports, law enforcement will describe evidence obtained via secret surveillance programs in inscrutable terms. “We’ve seen plenty of examples where the police officers in those reports write ‘we located the suspect based on information from a confidential source;’ they use intentionally vague language,” says Nathan Freed Wessler, a staff attorney at the ACLU’s Speech, Privacy, and Technology project. “It sounds like a human informant or something else, not like a sophisticated surveillance device.”

Sometimes, when a savvy defense attorney pushes, an unbelievable plea deal is offered, or the the case is dropped entirely. If a powerful, secret surveillance program is at stake, a single case is often deemed unimportant to the government.