Other radar devices have far more advanced capabilities, including three-dimensional displays of where people are located inside a building, according to marketing materials from their manufacturers. One is capable of being mounted on a drone. And the Justice Department has funded research to develop systems that can map the interiors of buildings and locate the people within them.
The radars were first designed for use in Iraq and Afghanistan. They represent the latest example of battlefield technology finding its way home to civilian policing and bringing complex legal questions with it.
Those concerns are especially thorny when it comes to technology that lets the police determine what's happening inside someone's home. The Supreme Court ruled in 2001 that the Constitution generally bars police from scanning the outside of a house with a thermal camera unless they have a warrant, and specifically noted that the rule would apply to radar-based systems that were then being developed.
In 2013, the court limited police's ability to have a drug dog sniff the outside of homes. The core of the Fourth Amendment, Justice Antonin Scalia wrote, is "the right of a man to retreat into his own home and there be free from unreasonable governmental intrusion."
Still, the radars appear to have drawn little scrutiny from state or federal courts. The federal appeals court's decision published last month was apparently the first by an appellate court to reference the technology or its implications.
That case began when a fugitive-hunting task force headed by the U.S. Marshals Service tracked a man named Steven Denson, wanted for violating his parole, to a house in Wichita. Before they forced the door open, Deputy U.S. Marshal Josh Moff testified, he used a Range-R to detect that someone was inside.
Moff's report made no mention of the radar; it said only that officers "developed reasonable suspicion that Denson was in the residence."
Agents arrested Denson for the parole violation and charged him with illegally possessing two firearms they found inside. The agents had a warrant for Denson's arrest but did not have a search warrant. Denson's lawyer sought to have the guns charge thrown out, in part because the search began with the warrantless use of the radar device.
Three judges on the federal 10th Circuit Court of Appeals upheld the search, and Denson's conviction, on other grounds. Still, the judges wrote, they had "little doubt that the radar device deployed here will soon generate many questions for this court."
But privacy advocates said they see more immediate questions, including how judges could be surprised by technology that has been in agents' hands for at least two years. "The problem isn't that the police have this. The issue isn't the technology; the issue is always about how you use it and what the safeguards are," said Hanni Fakhoury, a lawyer for the Electronic Frontier Foundation.
The Marshals Service has faced criticism for concealing other surveillance tools. Last year, the ACLU obtained an e-mail from a Sarasota, Fla., police sergeant asking officers from another department not to reveal that they had received information from a cellphone-monitoring tool known as a stingray. "In the past, and at the request of the U.S. Marshals, the investigative means utilized to locate the suspect have not been revealed," he wrote, suggesting that officers instead say they had received help from "a confidential source."
William Sorukas, a former supervisor of the Marshals Service's domestic investigations arm, said deputies are not instructed to conceal the agency's high-tech tools, but they also know not to advertise them. "If you disclose a technology or a method or a source, you're telling the bad guys along with everyone else," he said.
