NEW YORK • Los Angeles's transit agency is set to become the first in the United States to screen its passengers with body scanners as they enter the public transit system - a bold effort to keep riders safer from terrorism and other evolving threats.

But officials said that riders need not worry that their morning commute would turn into the sort of security nightmare often found at airports or even sporting events. In a statement on Tuesday, transit officials said the portable screening devices they plan to deploy later this year will "quickly and unobtrusively" screen riders without forcing them to line up or stop walking.

"We're looking specifically for weapons that have the ability to cause a mass casualty event," Mr Alex Wiggins, the chief security and law enforcement officer for the Los Angeles County Metropolitan Transportation Authority, said on Tuesday, according to The Associated Press.

"We're looking for explosive vests, we're looking for assault rifles. We're not necessarily looking for smaller weapons that don't have the ability to inflict mass casualties," he said.

The devices themselves resemble the sort of black laminate cases that musicians lug around on tour - not upright metal detectors. Mr Dave Sotero, a spokesman for Metro, said the machines, which are on wheels, can detect suspicious items from 9m away and can scan more than 2,000 passengers per hour. The units can be pointed in the direction of riders as they come down an escalator or into a station.

"Most people won't even know they're being scanned, so there's no risk of them missing their train service on a daily basis," he said. Mr Sotero said the agency had bought several units for about US$100,000 (S$138,000) each and that the authorities would be trained on how to use the technology.

The county's metro system has one of the largest riderships in the country, with 93 rail stations. Mr Sotero said the new scanning units would be mostly deployed at random stations, but will also be used in places where large crowds are expected for marches, races and other events.

The units, made by the company Thruvision, can detect hidden objects using technology that examines the naturally occurring waves produced by a person's body. The technology does not emit radiation and will not display a person's anatomy, officials added.

NYTIMES