U.S. Marshals secretly tracked 6,000 cellphones

The Marshals Service's surveillance log lists 5,975 cases in which the Marshals Service used stingrays. The agency declined to say what time period the log covered, or where the suspects were arrested. It also declined to identify the suspects, to protect their privacy.

"Just that sheer number is significant," American Civil Liberties Union lawyer Nathan Wessler said. "That's a lot of deployments of a very invasive surveillance tool."