Authorities ratcheted up security on the Los Angeles metro following a tip from overseas about an impending bomb attack Tuesday against a station in the sprawling rail network.
The threat was relayed by an anonymous man who called a public safety line run by an unidentified foreign government, which then passed on the information to a Federal Bureau of Investigation terrorism task force, said Deirdre Fike, assistant director in charge of the FBI's office in Los Angeles.
The target of the threatened attack was metro's Universal City station -- located near the Universal Studios theme park -- where the caller who spoke English said an explosive device would go off on Tuesday, Fike told reporters at a hastily organized press conference with local authorities.
Officials said they were working to determine the credibility of the threat described as "very specific."
"The threat is imminent and it's related to tomorrow" and concerned an off-ramp at the station, Los Angeles police chief Charlie Beck said.
He added that authorities had little time to assess the seriousness of the threat and because of its imminent nature had decided to move quickly and alert the public.
Los Angeles County Sheriff Jim McDonnell said the call could be real or a hoax and urged commuters to remain vigilant and report any suspicious activity.
He said undercover deputies had been deployed throughout the network, along with uniformed officers, members of the bomb squad and K9 explosive detection units.
Metro officials meanwhile tweeted a simple and succinct reminder to passengers, warning them to remain alert Tuesday.
"If you see something, say something," read the Twitter message sent from the railway network's @metrolosangeles handle.
Mayor Eric Garcetti said commuters should expect to be searched, but should go about their lives as normal. He said he planned to ride the metro on Tuesday from Universal City to reassure commuters.
"My advice is right now, everybody should go about their normal day tomorrow," Garcetti said.
He added that commuters should not be frightened by the heightened security.
"Those are there for your protection, and we meet threats with every possible precaution that we can," he said.
Authorities said there was added vigilance as the threat came about during the busy holiday season when many people were out shopping or would be visiting theme parks and attractions in and around Los Angeles, including Universal Studios and Disneyland.
The Los Angeles metro system consists of six lines, including two rapid transit subway lines and four light rail lines that serve 93 stations.
The Universal City station on the metro system's Red Line has been listed as among the busiest in the network.
Some 145,000 people use that line daily, Garcetti said.
The Red Line connects North Hollywood with the central and downtown Los Angeles.
Last year, an emailed threat forced the closure of Los Angeles schools for a day before it was determined to be a hoax.