A recently disclosed document shows the FBI telling a local police department that the bureau’s covert cell-phone tracking equipment is so secret that any evidence acquired through its use needs to be recreated in some other way before being introduced at trial.

“Information obtained through the use of the equipment is FOR LEAD PURPOSES ONLY,” FBI special agent James E. Finch wrote to Chief Bill Citty of the Oklahoma City Police Department.

The official notice, dated September 2014, said such information “may not be used as primary evidence in any affidavits, hearings or trials. This equipment provides general location information about a cellular device, and your agency understands it is required to use additional and independent investigative means and methods, such as historical cellular analysis, that would be admissible at trial to corroborate information concerning the location of the target obtained through the use of this equipment.”

The document, obtained by nonprofit investigative journalism outlet Oklahoma Watch, pertains to the use of cell site simulators, or Stingrays — surveillance technology that mimics a cellphone tower to trick cellphones into transmitting location data and other information, sometimes even the contents of calls.

Journalists and activists have uncovered at least 20 similar nondisclosure agreements between FBI and local police about Stingrays in the past few years — but the FBI’s advice about retroactively recreating evidence appears to be new.

Privacy advocates have long warned of “parallel construction,” in which investigators cover up information obtained without a warrant by finding other ways to attribute it — never allowing the source of the original lead to be scrutinized or subject to judicial oversight.

“This is the first time I have seen language this explicit in an FBI non-disclosure agreement,” Nate Wessler, a staff attorney with the American Civil Liberties Union’s Speech, Privacy, and Technology Project, wrote in an email to The Intercept. “The typical NDAs order local police to hide information from courts and defense attorneys, which is bad enough, but this goes the outrageous extra step of ordering police to actually engage in evidence laundering,” 

“Instead of just hiding the surveillance, the FBI is mandating manufacture of a whole new chain of evidence to throw defense attorneys and judges off the scent. As a result, defendants are denied their right to challenge potentially unconstitutional surveillance and courts are deprived of an opportunity to curb law enforcement abuses,” Wessler continued.

One concrete example of law enforcement engaging in parallel construction was the Drug Enforcement Agency’s “Hemisphere” program, in which agents were given access to troves of AT&T’s historical cell phone records and instructed to subpoena those same records to create a separate legitimate evidence trail.

Read the rest of the notice here: