Becoming the first mass transportation system in the United States to do so, the Los Angeles subway system will soon be using body scanners to screen its passengers for weapons and explosives.

According to a recent announcement, passengers entering subway stations in LA will be scanned, without having to pass through security checkpoints.

Using technology from U.K. manufacturer Thruvision, the scanners, which have a wide field of view, can screen passengers while they ride escalators or as they pass through ticket barriers. According to the makers of Thruvision, the technology is capable of detecting suspect items from as far away as 32 feet (10 m) as well as being able to scan as many as 2,000 passengers an hour.

Although screening will be voluntary, according to the report, those opting out of a scan will be prohibited from traveling via subway.

The system, for now, will be used on a pop-up basis at only a handful of stations throughout LA and will not be a permanent installation.

The technology works by using sensitive cameras to detect a person’s body heat through their clothing. If something is concealed under clothing, it will block body heat from being emitted, thereby signaling to authorities that something might be concealed. The decision to investigate is made by human operators inspecting those images.

David Pekoske, from the U.S. Transportation Security Administration (TSA), explained that the country faced "persistent threats to our transportation systems."

"Our job is to ensure security in the transportation systems so that a terrorist incident does not happen on our watch," he said.