A number of schools in the US city of Denver have been closed, after a woman said to be "infatuated" with the Columbine massacre bought a shotgun and ammunition.
The FBI has said Sol Pais is "armed and dangerous".
Jefferson County Sheriff's Office has said she traveled to Colorado on Monday night and "made threats."
The 18-year-old is described as white, approximately 5'5" in height and with brown hair.
She was last seen wearing a black t-shirt, camouflage pants and black boots.
She was last seen in the foothills of Jefferson County.
Sol Pais is armed and dangerous. We are operating out of an abundance of caution. The threat is not isolated to one school or individual. Federal, state, and local law enforcement are working together to keep our community safe. Tip Line: 303-630-6227. Media Line: 303-630-6427. pic.twitter.com/KENVT1kEK3
— FBI Denver (@FBIDenver) April 17, 2019
Jefferson County Sheriff's Office said they were investigating "what appears to be a credible threat possibly involving the schools."
"Children are safe. Deputies are at the schools. Multiple schools are on lockout in Columbine area and Mountains".
Saturday marks 20 years since two teenage gunmen - Dylan Klebold and Eric Harris - murdered 12 classmates, a teacher and then killed themselves at Columbine High School in a suburb of Denver.
At the time, it was America's deadliest school shooting.
Columbine High School itself and 21 nearby schools also put in place extra security on Tuesday, restricting entry and exit.
Dean Phillips, the agent in charge of the FBI in Denver, said the teenager was "extremely dangerous" but has not been charged with any crime.
He said Pais had made comments to others that made her a credible threat, but had not named a particular school.
"This has become a massive manhunt... and every law enforcement agency is participating and helping in this effort," Mr Phillips said.
The FBI's Rocky Mountain Safe Streets Task Force described Pais as "infatuated with (the) Columbine school shooting".
