DENVER — Hundreds of schools across the Denver area were closed on Wednesday as law enforcement officials engaged in a massive search for a Florida woman who they said had made threats ahead of the 20th anniversary of the Columbine High School shooting and was considered armed and “extremely dangerous.”

The decision to keep about half a million students home in two dozen school districts over a vast swath of Colorado showed the sense of alarm among the authorities. An F.B.I. bulletin sent to local law enforcement agencies on Tuesday identified the woman as Sol Pais, 18. She was “infatuated” with the Columbine attack, the bulletin said, and had been “attempting to buy firearms.”

Officials also released an image of Ms. Pais, and said she was approximately 5-foot-5, with brown hair. She had recently traveled to Colorado, they said.

Ms. Pais’ parents reported her missing to local police on Monday, Detective Sergeant Marian Cruz, a spokeswoman for the Surfside Police Department, said on Wednesday. That has been the only time police have been called to the family’s address, she added.