The radars work like finely tuned motion detectors, using radio waves to zero in on movements as slight as human breathing from a distance of more than 50 feet. They can detect whether anyone is inside of a house, where they are and whether they are moving.

Current and former federal officials say the information is critical for keeping officers safe if they need to storm buildings or rescue hostages. But privacy advocates and judges have nonetheless expressed concern about the circumstances in which law enforcement agencies may be using the radars — and the fact that they have so far done so without public scrutiny.
“The idea that the government can send signals through the wall of your house to figure out what’s inside is problematic,” said Christopher Soghoian, the American Civil Liberties Union’s principal technologist. “Technologies that allow the police to look inside of a home are among the intrusive tools that police have.”