Schools throughout the city of Denver, Colorado, have closed for the day as authorities search for an "extremely dangerous" woman.
Sol Pais, 18, is reportedly obsessed with the 1999 Columbine high school massacre, in which two teenagers murdered 12 students and a teacher.
The 20th anniversary of the attack is on 20 April.
FBI agent Dean Phillips told reporters Ms Pais was a "credible threat to the community".
Authorities issued a security warning to schools throughout the Denver area late on Tuesday. Since then, a number of districts have announced closures, including Cherry Creek, Mapleton and Jefferson County.
The CBS network reported that more than 130 schools had closed.
....
FBI agents in Miami alerted their colleagues in Colorado after Ms Pais travelled to the state.
While she made no specific threat against schools, Mr Phillips said her comments made her a danger. She has not been charged with any crime but the priority was to find her, he said.