At least 50 U.S. law enforcement agencies have access to radar devices that enable them to see through walls to detect whether anyone is inside, a practice that was otherwise unknown to the public until it surfaced last month in a federal appeals court. The device, Range-R, has raised new legal concern over government surveillance and privacy issues. A federal appeals court in Denver said in December that officers had used the device, which was first deployed more than two years ago, before entering a house to arrest a man who violated his parole. The judge said he was alarmed authorities had used the technology without a search warrant, and that “the government’s warrantless use of such a powerful tool to search inside homes poses grave Fourth Amendment questions.” Agencies including the FBI and the U.S. Marshals Service are equipped with the Range-R, which detects movement on the other side of a wall, but does not show a picture of what’s happening inside. Justice Department spokesman Patrick Rodenbush said officials are reviewing the court’s decision.