Many schools in the Denver area are closed Wednesday as authorities conduct what they're calling a "massive manhunt" for a young woman they say is "infatuated" with the 1999 shooting at Columbine High School. They say she made threats just days before the 20th anniversary of the attack that left 13 people dead.

Authorities are seeking Sol Pais, 18, who is thought to have made undisclosed threats that prompted Columbine and more than 20 other schools outside Denver to lock their doors for nearly three hours Tuesday afternoon.

All schools in the Denver vicinity were urged to tighten security because the threat was deemed "credible and general," said Patricia Billinger, a spokeswoman for the Colorado Department of Public Safety.

Get Breaking News Delivered to Your Inbox

Late Tuesday night, several districts announced schools would be closed Wednesday. Among them were Denver Public Schools and Jeffco Public Schools, short for Jefferson County, the district that includes Columbine High.

In collaboration with other Denver-metro area school districts, all Jeffco Public Schools will be closed on Wednesday, April 17 due to the ongoing safety concern. All facilities and programs are closed for the day – no employees are to report. — Jeffco Public Schools (@JeffcoSchoolsCo) April 17, 2019

Pais traveled to Colorado from Miami on Monday night and bought a pump-action shotgun and ammunition, according to the Jefferson County Sheriff's Office and the FBI. She was last seen in the foothills west of Denver, was considered armed and extremely dangerous, and should not be approached, they said.

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

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

The FBI's Rocky Mountain Safe Streets Task Force issued a notice Tuesday describing Pais as "infatuated with (the) Columbine school shooting." The alert also said police who come into contact with her should detain her and evaluate her mental health.

20 YEARS LATER: Columbine Legacy

Sheriff's spokesman Mike Taplin said the threats she made were general and not specific to any school.

CBS Miami reports Pais' family is asking her to turn herself in. The station talked through a closed door with a man identifying himself as her father at the family home in Surfside, Florida.

"She can, please, come home. I think she's going to be OK," the man said.

"It's like a bad dream. We don't know. We don't have any ideas," he said. "I think that maybe she has a mental problem."

The Denver Post reported that a call to the home was interrupted by a man who identified himself as an FBI agent who said he was interviewing Pais' parents. The agency was also speaking to neighbors.

Surfside Police Sgt. Marian Cruz said Pais was reported missing on Monday night by her parents. The Miami Herald and WTVJ-TV are reporting that she is a senior at Miami Beach High School.

Two teenage gunmen attacked Columbine on April 20, 1999, killing 12 classmates and a teacher.