In a handful of criminal cases around the country, local police officers have testified in recent months that non-disclosure agreements with the FBI forbid them from acknowledging the use of secret cellphone-tracking devices. In some, prosecutors have settled cases rather than risk revealing, during court proceedings, sensitive details about the use of the devices.

The FBI, however, says such agreements do not prevent police from disclosing that they used such equipment, often called a StingRay. And only as a “last resort” would the FBI require state and local law enforcement agencies to drop criminal cases rather than sharing details of the devices’ use and “compromising the future use of the technique.”

To date, the bureau hasn’t invoked that provision, FBI spokesman Christopher Allen said in a statement to The Washington Post.

The clarification sheds new light on the bureau’s policy on the controversial technology following months of reports about the growing use of the devices by state and local police without the express approval of judges. It also comes as the Justice Department is finalizing a review of federal law enforcement agencies’ use of the devices.

Cell-site simulators, also called international mobile subscriber identity (IMSI) catchers, are boxes about the size of a small suitcase that simulate a cellphone tower. They force mobile phones in their vicinity to transmit their phone numbers and unique electronic serial numbers. In so doing, they collect identifying information not just about a suspect’s phone but also about the phones of potentially hundreds of law-abiding citizens.

Law enforcement officials say the devices are valuable in tracking suspects whose whereabouts are unknown. Data released by police agencies, however, have shown the devices are used not only in cases of violent crime but in routine investigations into drug crimes and cellphone thefts.

“The FBI’s concern is with protecting the law enforcement sensitive details regarding the tradecraft and capabilities of the device,” Allen said in the statement.

But defense attorneys who have represented defendants in StingRay cases said the FBI assurances contrast with the way in which local police departments interpret the agreements they must sign to use the equipment.

“The reality is the FBI has made officers sign a non-
disclosure agreement that says they may not disclose any information about the technology in a trial,” said John Sawicki, a lawyer in Tallahassee, Fla., who has represented several defendants who were tracked by StingRay devices. “Then the bureau says none of the officers has come back to them to say, ‘Do I really have to follow that, rather than pushing for dismissal anyway?’ That’s a little disingenuous.”

Transcripts from court cases show that police officers, citing non-disclosure agreements, have refused even to acknowledge the use of the equipment.

In a November 2014 case in Baltimore, for example, when a police detective was quizzed by a defense attorney about his use of a cellphone-tracking device, he said: “I can’t discuss it sir. It’s a non-disclosure agreement.”

In Tallahassee in 2013, prosecutors reached a plea deal with a defendant accused of robbery with a deadly weapon rather than give up details of police officers’ use of a StingRay.

“This policy clarification from the FBI is welcome, but it rings hollow coming only after significant details of this technology have been outed by the press,” said Nathan Wessler, staff attorney for the American Civil Liberties Union.

The FBI also stated that it requires officers to obtain a search warrant to use the device, unless the case involves an emergency such as a kidnapping, missing child or crime that could lead to imminent death. The updated warrant policy was first reported by the Wall Street Journal.

The bureau had previously said it obtains court orders — which require a lesser standard than the probable cause required for a warrant — to use the devices. Judges, however, typically are not informed by the law enforcement agencies that they are planning to use a cell-site simulator. Instead, orders are often granted for the purpose of obtaining cell-tower information from a phone company. In fact, an agency will use such tower data to identify an area within which to home in on a suspect using the StingRay or other tracking device.

In Washington state on Monday, Gov. Jay Inslee (D) signed into law a measure that requires a warrant for the use of cell-site simulators, accompanied by an application describing the type of device to be used, the target phone number and the geographic area to be covered. The law also orders the police to delete any information collected from any bystander who is not a target.