In a growing trend begun a few years ago without much public disclosure or debate, at least 50 police departments across the country — including the FBI and U.S. Marshals Service — now deploy radar devices that could allow them effectively to “see through walls.”

The technology consists of hand-held devices that send out radio waves which can detect the slightest movement, even breathing, from distances up to 50 feet away. Other advancements currently in development could be mounted on drones and other vehicles and could possibly map out a building’s interior and locate people inside it.

These systems, which first appeared on the battlefields of Afghanistan and Iraq, have been the subject of significant investment by the Justice Department. Law enforcement experts believe the devices could be valuable tools to keep officers safe when entering a building to arrest suspects or rescue hostages. Of course, they also have attracted the attention of civil liberties advocates and even some federal courts. Civil libertarians understandably worry that police departments could use the devices without a search warrant, potentially in violation of the Fourth Amendment’s protections against unreasonable search and seizure.

The devices have been the subject of least one federal appellate Courts ruling thus far, when the 10th Circuit last month upheld the conviction of a man for a parole violation and firearms charges, after he was apprehended by U.S. Marshals who used the devices. While acknowledging the radar system posed troubling Fourth Amendment questions, the court ultimately punted on those, noting that, in executing the suspect’s arrest warrant, marshals already had other evidence establishing probable cause that he was in the residence.

Should questions about the technology ever reach the U.S. Supreme Court, which seems terribly likely, there are at least two similar cases that might provide some guidance. In 2001, the high court ruled that police needed a warrant to scan the outside of a house with a thermal camera. In a 2013 case involving a decidedly less high-tech form of surveillance, the court put restrictions on law enforcement’s use of drug dogs to sniff outside a house.

In the end, the relevant public policy questions lie not in the technologies themselves. There’s good reason to believe these hand-held sensors could improve safety, both for police officers and for the general public. One of the concerns with so-called “no-knock” raids is that frequency with which innocent people are harmed or even killed, and this technology could alert law enforcement to the presence of innocent civilians or counsel them to pursue less risky means of apprehension.

Nonetheless, with the potential both for harm to civil liberties and increased safety for both police and civilians are like, state legislatures and Congress should examine whether regulations on this sort of technology are appropriate, before these cases do wind up in the courts. Some guidelines might include

These must be considered among the most intrusive tools available to law enforcement, but if used properly, they could make policing less dangerous for civilians. It’s up to state legislatures and Congress to craft radar usage regulations that protect both civil liberties and the lives of police officers and ordinary civilians.



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