The judges expressed alarm that agents had used the new technology without a search warrant, warning that “the government’s warrantless use of such a powerful tool to search inside homes poses grave Fourth Amendment questions.”

At least 50 U.S. law enforcement agencies have secretly equipped their officers with radar devices that allow them to effectively peer through the walls of houses to see whether anyone is inside, a practice raising new concerns about the extent of government surveillance.

Those agencies, including the FBI and the U.S. Marshals Service, began deploying the radar systems more than two years ago with little notice to the courts and no public disclosure of when or how they would be used. The technology raises legal and privacy issues because the U.S. Supreme Court has said officers generally cannot use high-tech sensors to tell them about the inside of a person’s house without first obtaining a search warrant.

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.

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