A passenger is scanned during a demonstration at Union Station in Los Angeles on Aug. 14. (Richard Vogel/AP)

“Passengers proceeding past this point are subject to Metro security screening and inspection.”

Those were the words on a sign posted next to a new portable body scanning machine being used on people entering the Los Angeles Metro at Union Station Tuesday, the Los Angeles Times reported.

It is one of several body scanners purchased by the Los Angeles County Metropolitan Transportation Authority in partnership with the Transportation Security Administration (TSA), priming the city to be the first in the United States to install the technology, according to a Metro news release.

Created by Thruvision, a company based in the United Kingdom, the machines will scan people for weapons and improvised explosive devices “intended to cause mass casualties,” the release said. They are capable of detecting metallic and nonmetallic objects, according to transit officials.

“This new technology will augment our aggressive safety and security posture and help us proactively deter potential attacks to our system,” Sheila Kuehl, L.A. County supervisor and Metro board chair, said in the release.

Echoing Kuehl at a news conference Tuesday where one of the machines was rolled out and demonstrated, TSA Administrator David Pekoske said, “We will not have a repeat of 9/11 or any terrorist incident inside our transportation systems in the United States.”

Metro spokesman Dave Sotero told The Washington Post in an email that each scanner costs about $100,000 and can screen more than 2,000 passengers an hour. The scanning process will not invade privacy or cause delays for riders, Sotero wrote.

“It is a passive, unobtrusive scanning technology that does not reveal any anatomical details and does not interfere with foot traffic at our transit stations,” he wrote.

Positioned in front of an escalator at Tuesday’s event, the scanner’s display featured a split screen: One side was a live video feed, while the other showed the person being scanned as a glowing green figure — “generic avatars,” officials call them. The machine itself is housed in a black and chrome trunk-like case that has wheels, meaning it can be deployed and used at any of the Metro’s 93 subway and light-rail stations.

The device, which does not emit any type of radiation, searches for weapons by examining the “naturally-occurring waves produced by a person’s body,” according to the Metro statement. If a person has concealed an object under clothing or has something strapped to their body, a black spot or color indicator will appear on the computer-generated avatar.

The idea is to detect weapons such as explosive vests and assault rifles, said Alex Wiggins, the chief security and law enforcement officer for the Los Angeles County Metropolitan Transportation Authority, according to the Associated Press.

“We’re not necessarily looking for smaller weapons that don’t have the ability to inflict mass casualties,” Wiggins said.

However, the L.A. Times reported that Wiggins declined to comment on whether a person carrying a handgun would be subject to an additional search. He also declined to comment on what the response would be if a scanner finds a possible weapon, according to the Times.

On the one hand, officials said the screening process is not mandatory. But if people choose not to be scanned, they could be barred from entering the station, Wiggins said.

“That means not taking transit that day,” he said.

While Los Angeles may be the first city to deploy the scanners, the same technology as well as other types of screening devices have been tested at stations around the country, including New York’s Penn Station, D.C.’s Union Station and a New Jersey Transit station, AP reported.

Most recently, on Tuesday, the Port Authority partnered with TSA to test out a scanner at a Port Authority Bus Terminal in Manhattan, WPIX reported.

According to the news station, the device, a tripod-mounted camera connected to a computer, is called a “standoff explosive detection unit.”

Counterterrorism experts have long pointed out that train and rail systems are vulnerable to attack because of a lack of heavy security or exhaustive screening methods. But securing passenger railways is a “daunting task” given the ” abundance of passengers, combined with the need for easy access,” security analyst Eben Kaplan wrote in a 2007 article published by the Council on Foreign Relations.

“Absolute security can never be achieved, and experts caution against extreme security measures, which they say would disrupt how transportation systems function while offering no guarantee against attack,” Kaplan wrote.

While measures such as more security officers and bomb-sniffing dogs, random searches, and increased video surveillance have been used in an attempt to prevent attacks, “they would be unlikely to foil a determined terrorist cell,” the article stated.

In London last year, 29 people riding on the subway were injured when a bomb “partially exploded” in a train car. The attack, known as the Parson Green train bombing, was classified by authorities as a terrorist incident.

A few months later in New York, a 27-year-old man entered a Manhattan subway station with a crude pipe bomb strapped to his body, The Post reported. The bomber detonated the device while walking in a subway passage, injuring himself and three others.

According to the L.A. Times, the new scanners will be randomly placed at Metro stations around the city and used to respond to threats of terrorism or to screen “large crowds at a station near a protest or a sporting event.”

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

The increased screening on public transit comes as the TSA is considering eliminating security checkpoints at more than 150 smaller U.S. airports.

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