The FBI is investigating an imminent threat against the Los Angeles Metro system for which it had received specific information, officials said.
Authorities were still trying to determine if the threat was credible.
The agency received information about the threat for tomorrow morning against the red line Metro.
According to the FBI, an international partner had given the agency information about an anonymous call to a public safety line reporting a threat, not making one.
Officials decided to alert the public because of the imminence and specificity of the threat.