Schools throughout the city of Denver, Colorado will shut on Wednesday as authorities search for an "extremely dangerous" woman, BBC reports.
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.
"All facilities and programs are closed for the day - no employees are to report," Jefferson County Public Schools posted on Twitter, saying they made the decision "in collaboration with other Denver-metro area school districts".