Identified as Sol Pais, 18, the woman had traveled to Denver and bought a firearm ahead of the 20th anniversary of the Columbine High School shooting, officials said.

Authorities said Pais was considered armed and “extremely dangerous,” and the decision to keep about half a million students home in two dozen school districts showed the sense of alarm among officials.

An FBI bulletin sent to local law enforcement agencies Tuesday said Pais was “infatuated” with the Columbine attack, and officials expressed concerns about her mental stability. She had also purchased a shotgun and ammunition after arriving in Denver, authorities said.

Pais, a student at Miami Beach Senior High School, had last been seen wearing a black T-shirt, camouflage pants and black boots, authorities said.

In a news conference Tuesday night, Dean Phillips, the special agent in charge of the FBI’s office in Denver, said the search had turned into a “massive manhunt.” Phillips said his team had received a tip Tuesday morning from federal agents in Miami identifying Pais as a possible threat in Colorado.

School superintendents throughout the Denver area decided during a conference call Tuesday night to jointly close schools Wednesday morning as a precaution, The Denver Post reported.

Pais’ parents reported her missing to local police Monday, Detective Sgt. Marian Cruz, a spokeswoman for the Surfside Police Department, said Wednesday. Police have not been called to the family’s address before, she added.

Daisy Gonzalez-Diego, a spokeswoman for the Miami-Dade County Public Schools, confirmed Wednesday that Pais is a student at Miami Beach Senior High School. The school district is assisting the FBI with its investigation, Gonzalez-Diego said.

The FBI then discovered that Pais had arrived at the Denver airport and bought a pump-action shotgun and ammunition at a store. “She was then taken to an area where she was last seen out toward the foothills,” Phillips said.

“Because of her comments and her actions, because of her travel here to the state, because of her procurement of a weapon immediately upon arriving here,” he added. “We consider her to be a credible threat — certainly to the community and, potentially, to schools.”

The search for Pais quickly upended families across the Denver region. Thousands of parents woke Wednesday morning to discover that schools had been canceled and that they would have to explain the cancellation to their children.

For some of the youngest students, this was their first introduction to the Columbine shooting, and to its legacy.

Some parents decided that they would keep their children inside all day; others said this would effectively hand Pais a victory.

“It’s sad and scary,” said Jeff Desserich, a math teacher at a charter school in Denver, who spent the morning trying to explain to his daughters Anais, 8, and Elena, 6, why they would not be going to class.

“I said, ‘There is a lady, she probably has some sort of mental health issue,’” he said, “And I talked a little about the sad events of Columbine and how her flying to Denver and buying a weapon, that’s a really big flag for law enforcement.”

Just last Friday, Colorado’s Democratic governor signed a “red flag” law that would allow guns to be temporarily seized from people deemed to be dangerous to themselves or others. The act was bitterly opposed by more than a dozen sheriffs and officials from largely rural, conservative counties who vowed not to enforce it.

The state also passed significant gun control measures in 2013 that expanded background checks, but despite that, Colorado does not have a specific waiting period for someone who wants to buy a gun.

In Florida, The Miami Herald reported that a man who answered the door at Pais’ address Tuesday identified himself as her father and said he had lost contact with her Sunday. “I think maybe she’s got a mental problem,” he told The Herald. “I think she’s going to be OK.”

In Colorado, the announcement prompted “lockouts,” or heightened security measures, at schools in Jefferson County and the surrounding area Tuesday. During a lockout, all exterior doors are locked at a school but business continues as usual inside. Police officers aided in end-of-day student release. County officials said that all students and staff members were safe.

It was not the first threat for students at Columbine High School. In December, an anonymous caller claimed bombs had been planted inside the school. Police responded, but the threat proved to be a hoax.

During the Columbine High School shooting on April 20, 1999, two students shot and killed 12 of their classmates and a teacher.

The shooting’s aftermath was widely televised, and young people across America continue to be influenced by the symbology of the Columbine shooting and the students who carried it out, according to law enforcement officials, researchers and educators.

In May 2018, a 17-year-old junior in Santa Fe, Texas, shot his teachers and fellow students with a sawed-off shotgun while wearing a black trench coat and carrying Molotov cocktails, his arsenal and attire inspired by the Columbine gunmen. The 20-year-old attacker who killed 26 people at Sandy Hook Elementary School in Connecticut in 2012 had compiled materials on the Columbine attackers on his computer. And in his manifesto, the 23-year-old student who shot and killed 32 people at Virginia Tech in 2007 had called the Columbine gunmen by their first names and described them as “we martyrs.”

The killers have achieved dark folk hero status in the corners of the internet where their carefully planned massacre is remembered, studied and in some cases even celebrated, officials say. Their admirers, often known as “Columbiners,” are frequently depressed, alienated or mentally disturbed, drawn to the Columbine subculture because they see it as a way to lash out at the world and to get the attention of a society that they believe bullies, ignores or misunderstands them.

Jefferson County, home to Columbine High School, has spent the past 20 years grappling with that legacy.

Students, teachers, families and law enforcement officers have had to deal not only with the emotional trauma of the shooting, but also with the people who have become obsessed with it and the copycats who have carried out their own attacks.

In an interview last year, the head of safety for Jefferson County schools, John McDonald, said he had often apprehended people who came from around the country to try to enter the school, a major safety concern. These visits — and interest in the shooting — have only increased over time, he said: “I’ve been dealing with this for more than a decade, and it’s never been more of an issue than it is now, 20 years later.”

This article originally appeared in The New York Times.