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.
"All facilities and programmes are closed for the day - no employees are to report," Jefferson County Public Schools posted on Twitter, saying they had made the decision "in collaboration with other Denver-metro area school districts".
Jefferson County sheriff's office earlier posted photos of the woman on Twitter.
She is described as an 18-year-old white female from Florida last seen wearing camouflage trousers, a black T-shirt and black boots.
"She is armed and considered to be extremely dangerous," the sheriff's office post reads.
A tragedy that cuts deep
Nada Tawfik, BBC News, Denver
Anniversaries can be difficult for survivors and the wider community, often triggering traumatic memories.
Littleton was already on high alert because they've faced threats in the past, and this was another scary reminder that the mass shooting will perhaps forever affect life in this city. One Columbine survivor told me that when she heard the news, she questioned her safety and took a different route home in a moment of panic.
Today, schools are closed, empty, and deemed unsafe. John McDonald is in charge of security for the school district that includes Columbine. He told me he was angry that as a country, Americans had done nothing but point fingers for the last 20 years, and as a result children weren't just dead, they were unsafe to attend school.
Columbine isn't even among the ten deadliest mass shootings in this country any more. Yet, it is still a tragedy that cuts deep and becomes more painful with each reminder that nothing has changed.
The suspect's family has urged her to turn herself in.
"It's like a bad dream. We don't know. We don't have any ideas," her father told CBS Miami.
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.
What happened at the Columbine massacre?
- Took place on 20 April 1999 at Columbine High School
- Eric Harris and Dylan Klebold murdered 12 students and a teacher before killing themselves
- At the time it was the deadliest US high school shooting in history
- Michael Moore made the documentary Bowling for Columbine about the shooting in 2002