LITTLETON, Colo. (AP) — Denver-area public schools closed Wednesday as the FBI hunted for an armed young Florida woman who was allegedly "infatuated" with Columbine and threatened violence just days ahead of the 20th anniversary of the attack.

Sol Pais, an 18-year-old Miami Beach high school student, flew to Colorado on Monday night and bought a pump-action shotgun and ammunition, the FBI and the sheriff's department said.

All classes and extracurricular activities for about a half-million students were canceled as a precaution. Sheriff's spokesman Mike Taplin said the young woman's threats were general and not specific to any school.

"This has become a massive manhunt ... and every law enforcement agency is participating and helping in this effort," said Dean Phillips, agent in charge of the FBI in Denver.

The FBI said Pais is "considered to be extremely dangerous" and "made threats to commit an act of violence in the Denver metropolitan area" ahead of Saturday's anniversary of the 1999 shooting at Columbine High School that killed 13 people.

The FBI described Pais as "infatuated" with the Columbine school shooting.

Authorities said Pais was last seen not far from Columbine — in the Jefferson County foothills outside Denver — in a black T-shirt, camouflage pants and black boots. They appealed for anyone seeing her to call an FBI tip line at (303) 630-6227.

The alert also said police who come into contact with her should detain her and evaluate her mental health.

Because of the threat, Columbine and more than 20 other schools outside Denver locked their doors for nearly three hours Tuesday afternoon, and some canceled evening activities or moved them inside.

"We always have heightened awareness close to high-profile anniversaries like this," said Patricia Billinger, a spokeswoman for the Colorado Department of Public Safety.

Messages left by The Associated Press at two numbers listed for Pais' relatives in Florida were not immediately returned, while another number was disconnected.

Two teenage gunmen attacked Columbine on April 20, 1999, killing 12 classmates and a teacher before taking their own lives.

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Associated Press writer Thomas Peipert in Denver contributed to this report.