By Richard Winton, Los Angeles Times (TNS)

There are few places where Southern Californians feel more trapped than on a freeway in standstill traffic.

One of the San Bernardino shooters and his childhood friend talked about taking advantage of this vulnerability by launching a terrorist attack on a clogged freeway using guns and pipe bombs, according to court records released last week.

Syed Rizwan Farook and Enrique Marquez Jr. mapped out the plan to attack the 91 Freeway in detail and collected weapons and explosives in 2011 and 2012, only to abort the plot, the records say.

Still, their alleged scheme has counterterrorism experts alarmed and baffled. Experts say such an attack would be feasible, but some questioned the seriousness of the plot, given that there are so many easier ways for shooters to kill large numbers of people.

Officials said they had never uncovered an alleged terrorist plot involving freeways until now. But Los Angeles Police Department Deputy Chief Michael Downing, who oversees the counterterrorism bureau, said it’s a scenario officials have discussed and tried to plan for.

It is “a nightmare that we worry about,” Downing said.

According to a criminal complaint filed against Marquez last week in federal court, he and Farook planned to target a location on the 91 Freeway during the afternoon rush hour.

The complaint does not specify the exact location but said the area lacked exits, which the two believed would increase the number of targets. Marquez admitted to authorities that he would set up a position in the hills south of the freeway as Farook would throw pipe bombs into the eastbound lanes to stop traffic, the court papers said.

Farook would then move among the stopped vehicles, shooting motorists. Marquez said his plan was to shoot at stopped vehicles from the hill and watch for approaching police and emergency workers, prosecutors alleged. His priority, he said, was to shoot at police before firing on medical personnel.

The document said Marquez stopped planning the attack in 2012 for several reasons, including the arrest of a group of men in Chino in an unrelated terrorism case.

Downing said that even staging an accident on a section of a freeway with few exits would give terrorists the ability to attack motorists stopped in traffic.

The key for law enforcement, he said, would be to send officers to the scene quickly to stop the attack.

Farook and his wife, Tashfeen Malik, killed 14 people Dec. 2 at a holiday party in San Bernardino. They were later killed in a shootout with authorities. Both are said to have expressed support for Islamic radicalism. Marquez was charged last week with conspiring to provide material support to terrorists related to the 91 Freeway plan, and another aborted plot targeting Riverside City College. He was also charged with illegally providing some of the weapons used in the Dec. 2 shootings.

Both the San Bernardino shooting and the Islamic State attacks in Paris underscored the vulnerabilities of “soft targets.” Both attacks occurred away from landmarks that officials have often considered the most likely terrorist targets and instead hit suburban areas with much more limited security.

The alleged 91 Freeway plan is one of many plots uncovered in the United States that targeted transportation. According to the New York Police Department, plans included cutting the support cables of the Brooklyn Bridge, exploding bombs in the New York subway system and blowing up train tubes under the Hudson and East rivers.

There have been several aborted efforts to commit terrorism at transportation hubs, law enforcement authorities say:

2000: Plot uncovered to attack Los Angeles International Airport.

2003: Man plots to cut cable supports for the Brooklyn Bridge.

2006: Group talks of blowing up a train tunnel under the Hudson River, causing massive flooding.

2006: Men plot to blow up gas storage tanks and lines leading to JFK Airport.

2008: Man talked of using a suitcase bomb in river tunnel on Long Island Railroad.

2009: Men plot bombings in New York subway system.

2012: Men are accused of plot to blow up a bridge near Cleveland.

Source: Los Angeles Times reports; New York Police Department

©2015 Los Angeles Times. Distributed by Tribune Content Agency, LLC.

Photo: Raymond Shobe via Flickr