los angeles red line
Commuters board a train a Metro Red Line subway train at Union Station in downtown Los Angeles, CaliforniaReuters

The FBI is investigating a threat against the Los Angeles Metro system, which was given to them by a foreign caller, the FBI and Los Angeles officials said on Monday. They were alerted about a "specific" threat against the Red Line commuter railway system, which led to increased security. Alerts were also sent to the public.

"This threat is imminent, ... it is very specific," Los Angeles police chief Charlie Beck told an evening news conference. "But the credibility still needs to be vetted."

The FBI said that the threat was made on Monday morning and they reached out to the Los Angeles police department immediately. The LAPD has prepared response for such an incident. They also said that there is increased threat posture during holidays.

The law enforcement agency of another country, which was not identified, had given the warning since the threat had originated in their country, the police officials were quoted as saying by Daily Mail. 

The caller, who was male and spoke English, warned the US' international partners against the Red Line station across from the Universal Studios theme park on Tuesday, Beck said. The caller was not threatening to carry out the attack but said he was alerting the police. 

Police presence will be increased at the Universal City/Studio City station and elsewhere along the city's rail lines, Beck said. 

"You will see additional measures but they are out of an abundance of caution," Beck said.

Mayor of Los Angeles Eric Garcetti said he would board the train at the Universal City station as a show of confidence. 

"Everybody should go about their normal day tomorrow," Garcetti said. "People may see heightened security ... But don't let those (additional officers) frighten you."

The specific metro line, mentioned in the threat, carries almost 1,45,000 people every day. It travels between downtown Los Angeles and northern neighbourhoods, including Hollywood and North Hollywood.