Denver-area schools were closed on Wednesday as authorities searched for a woman they said was a “credible threat” to the community, a scare that comes just days before the 20th anniversary of one of the country’s deadliest school shootings at Columbine High School.

Authorities identified the suspect as 18-year-old Sol Pais of Surfside, Fla. They say she flew to Colorado from Miami and procured a pump-action shotgun and ammunition upon arriving. She is considered “extremely dangerous” and was last seen in the county’s foothills, clad in camouflage pants, black boots and a black T-shirt.

“This has become a massive manhunt,” said Dean Phillips, head of the Denver office of the FBI, which is leading the investigation.

Phillips said Pais has shown an “infatuation” with the 1999 shooting at Columbine, an event that still attracts intense attention and adoration in fringe online communities.

FBI agents were seen entering Pais’s home in South Florida Tuesday night. A man who identified himself as her father told the Miami Herald of his daughter: “I think maybe she’s got a mental problem."

Pais’s parents reported the teenager missing Monday night, after losing contact with her on Sunday.

“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,” Phillips said, “we consider her to be a credible threat — certainly to the community and, potentially, to schools.”

The threat, which the FBI said was “not isolated to one school or individual,” led county public school officials on Tuesday afternoon to place Columbine and nearby schools on “lockout,” which means classes continue as usual inside while entries to and exits from the building are restricted.

As the investigation continued into early Wednesday, classes were canceled and many schools decided not to open at all. The announcement of closures in metro-area school districts came in a tweet from the Colorado Department of Education shortly before midnight. The state agency said individual districts would provide details about closures.

In addition to Denver Public Schools, which serves nearly 100,000 students across roughly 200 schools, numerous other school districts said they would not open on Wednesday as a precaution.

These included Aurora Public Schools and Cherry Creek Schools, as well as Jefferson County Public Schools, which includes Columbine High School. The school district in Jefferson County, which enrolls 86,000 students, pointed to the “ongoing safety concern,” instructing all students and staff to stay home for the day.

“We understand that the decision to close schools tomorrow will have a significant impact on families, but we believe this is the right decision to ensure the safety of our students and staff,” Cherry Creek Schools stated in a tweet. “Safety is the priority!” declared the superintendent, Scott Siegfried.

On Tuesday, law enforcement said students would remain in school through the day. Extra precautions were taken at schools near Columbine, where sheriff’s deputies were on hand.

Caution spread to schools throughout the Denver area, which also issued lockouts or said they were monitoring the situation.

Two hours after announcing the lockouts, the school system reported that all students and staff were safe, adding that students would be released from the schools and buses would run on their normal schedule, though some may be slightly delayed. Officials said extra security would be present on the affected campuses.

After-school activities and sports practices also continued as scheduled — except at Columbine, where they were canceled for the day, the school said.

Since April 20, 1999, when two gunmen stormed Columbine, killing 13 people and wounding 24, threats of violence have become a painful fact of everyday life for the high school and the highly trained security team tasked with keeping it safe.

“It’s certainly not the first threat we’ve had that involves Columbine High School,” said Jefferson County Sheriff Jeff Shrader at the Tuesday briefing. “I know that this opens a wound, especially on an anniversary week, for those families that were most deeply impacted by this.”

The 20th anniversary of the shooting at Columbine is Saturday. As the district prepares for the day’s memorial events, it is fending off an onslaught of curious strangers who trespass in the school’s parking lot — sometimes more than 30 people in a day. Some say they just want to pay their respects, but others claim they are in love with the shooters — or sometimes that they have been reincarnated with the shooters’ souls.

The district has also seen an increase in threats and concerning messages, which often come in the form of emails to the school or phone calls to the 24-hour dispatch center run by the district’s security team.

The frequency of threats means the 1,700 students at Columbine are accustomed to lockouts, when the exterior doors of the building are locked, and lockdowns, when the interior classroom doors are secured.

The safety unit run by Jefferson County Public Schools is considered one of the most sophisticated school security systems in the country. Officials emphasize taking every threat — no matter how vague — seriously.

That system was tested in December, when Columbine High School went into lockout for hours on a Thursday. Two threats were made to the school: One call said there was a bomb inside the building, and another said there was a person with a gun outside. The threats were quickly investigated and found to be not credible.

Mark Berman contributed to this report.