Advanced cell phone tracking devices known as StingRays allow police nationwide to home in on suspects and to log individuals present at a given location. But before acquiring a StingRay, state and local police must sign a nondisclosure agreement with the FBI, according to documents released via a MuckRock FOIA request. As Shawn Musgrave reports, it's an unusual setup arrangement for two public agencies to swear each other to secrecy, but such maneuvers are becoming more common.