It’s been a bad week for Stingray secrecy. Following a court-ordered document dump in New York earlier this week, a Baltimore detective yesterday testified in court that he had personally used a Stingray between 600 and 800 times during two years as a member of the Baltimore Police Department’s Advanced Technical Team. He also testified that the unit has used such devices 4,300 times since 2007.

Stingrays are handheld or vehicle-mounted surveillance devices that operate by mimicking cell towers. They have the capability to force cell phones within their range to connect with the Stingray and transmit ID information from the phone. Some models — the technology is constantly being upgraded to keep pace with advancing telecommunications infrastructure — are suspected of being able to intercept content, but the true extent of the capability is a closely-guarded secret. What is increasingly not a secret is that dozens of law enforcement agencies around the country have been using these devices for years to sweep up swaths of cell phone data, much of it from innocent people, with little to no transparency or oversight.

The Baltimore detective refused to produce the device in court, citing an FBI non-disclosure agreement. The FCC, which regulates radio-emitting devices like Stingrays, has delegated to the FBI the authority to set conditions on local use of cell site simulators. The FBI, in turn, produced an agreement so restrictive that police and prosecutors can be obligated to withdraw evidence or even drop charges rather than disclose the use of the devices to the court.

As more and more information about these devices and their uses by law enforcement trickles out, it’s worth questioning what value exists in these secrecy agreements. Despite repeated references to “terrorists” and “national security” as a means for maintaining secrecy about Stingray use, the data that has been released detailing the purposes of actual Stingray investigations — such as this breakdown from the Tallahassee Police Department that contains not a single terrorism reference — suggests that Stingrays are used virtually entirely for routine law enforcement investigations. Meanwhile, the sacrifices being made in the name of defeating terror impose a real cost.

A quote from the Baltimore Sun is instructive:

In Maryland U.S. District Court last fall, an argument about the stingray device was cut short when the suspects took plea deals. And on Wednesday, following Cabreja’s testimony, prosecutors and defense attorneys entered into plea negotiations instead of debating the merits of the stingray further.

In cases where the stingray becomes a sticking point, Wessler said, “defense attorneys are being able to get really good deals for their clients, because the FBI is so insistent on hiding all of these details.”

“There are likely going to be a lot of defense attorneys in Baltimore who may have an opportunity to raise these issues,” Wessler said. “They are on notice now that their clients may have some arguments to make in these cases.”

It stands to reason that at least some of the suspects receiving sweetheart deals from prosecutors as a result of FBI meddling are actually guilty of serious criminal behavior. They’re getting deals (or having their cases dropped entirely) solely as a means of silencing courtroom discussion about Stingray use. Thus secret law enforcement Stingray use not only threatens the Constitutional rights of everyone swept up in the signal dragnets and the rights of suspects who are charged (and presumably have been convicted) based on improper evidence, but it also undermines a fundamental principle of criminal law: bringing actual criminals to justice.

Not only do these secret tactics jeopardize trials, they also open the door to suits from those convicted with improperly collected evidence. Just this week, following the document dump in Erie County, New York, the chief attorney of Legal Aid Buffalo declared that he had never heard of Stingrays and will now be investigating to see how they’ve affected his clients.

As more defense attorneys and judges become aware of the widespread use of this technology, more police officers will inevitably be put in the position of violating the non-disclosure agreement or forcing prosecutors to withdraw important evidence from serious criminal cases.

That is an unacceptably steep price to pay just so the federal government can shield an increasingly open secret from public oversight.