The threat was about something that would happen on Tuesday at the Universal City Red Line station, according to the FBI. The male caller was reporting the threat, not making one himself, officials said.
The threat was telephoned early Monday morning to an anonymous law-enforcement line in a foreign country. International partners with the FBI's Joint Terrorism Task Force relayed the threat and officials in Los Angeles decided to step up security while they work to determine if the threat was credible.
"This could be real, it could be a hoax, but we must remain calm but vigilant," Los Angeles County Sheriff Jim McDonnell said.
Commuters at the station said they were worried, with one person saying he would not ride the train Tuesday. But other said they would continue to ride the trains.
Specific details about the nature of the threat were not released.
Mayor Eric Garcetti said he was confident in law enforcement's response to the threat and plans to ride the Red Line Tuesday as a show of support. He added people should remain alert.