Los Angeles will be the first U.S. city to deploy body scanners, similar to those used by airport security, in its sprawling transit system.

The city last fall partnered with the federal Transportation Security Administration (TSA) to test a portable body scanner at one of its busiest light rail stops, the 7th Street Metro Center Station. On Tuesday, Los Angeles announced that it is making the scanners a permanent part of its transit security.

Because they are portable, the scanners will be moved around from location to location. Passengers should not expect to see the devices at every station or every day. Los Angeles Metro said the scanners would “augment” its existing security measures.

While Los Angeles is the first city to deploy the devices, others have been testing them.

In 2014, New Jersey Transit tested a similar technology in the run-up to the Super Bowl. So far this year, TSA has partnered with New York City to test scanners in Pennsylvania Station; with Amtrak to test-screen rail passengers at Union Station in Washington, D.C.; and with the San Francisco Bay Area Rapid Transit District. Earlier this week, the security administration announced more testing at New York City's Port Authority Bus Terminal.

Unlike full-body scanners at airports, these portable systems are designed to quickly scan passengers moving through a station rather than forcing those passengers to stop at a checkpoint. The machines scan for metallic and non-metallic objects on a person’s body and are capable of detecting a suspicious item from 30 feet away. They can scan more than 2,000 passengers per hour, according to TSA.

“In surface transportation or mass transit, a checkpoint is not a feasible way to screen people," says TSA spokesperson Lisa Farbstein. "The whole point of mass transit is to get people from point A to point B quickly."

Since 9/11, numerous new security measures have been put in place in the nation's airports. But ground transit security has lagged. Officials say the new body scanners are intended to help close the security gap.

“We’re dealing with persistent threats to our transportation systems in our country,” TSA Administrator David Pekoske said at press conference Wednesday. “Our job is to ensure security in the transportation systems so that a terrorist incident does not happen on our watch.”

The scanners have caused concern among some privacy advocates. The scanners will show where voids or masses -- potentially a weapon or large object -- is located on a person, which may not be enough for a police officer to establish probable cause and conduct a search.

"That strikes me as an incredibly crude way of trying to work out whether people are carrying dangerous devices," Eric Miller, a professor at Loyola Law School in Los Angeles told local public radio last year. "It only benefits us if there's a lot of other evidence that would suggest that a particular station is currently vulnerable or not. Otherwise they're just doing mass data collection of individuals."