Authorities said Tuesday they are looking for an 18-year-old woman suspected of making threats against Columbine High School, just days before the 20th anniversary of a mass shooting that killed 13 people.

The information prompted a lockdown at the high school and several others outside Denver. All students were safe, school officials said.

Sol Pais traveled to Colorado on Monday night and made threats against the schools, the Jefferson County Sheriff's Office and the FBI said. Officials didn't provide further details about the threats or say where she was from.

Pais was last seen in the foothills west of Denver, was considered armed and extremely dangerous and should not be approached.

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She is reportedly 'infatuated' with the 1999 massacre.

The doors were locked at Columbine and more than 20 other schools in the Denver area as the sheriff's office said it was investigating threats against schools related to an FBI investigation.

Students left classes on time, but after-school activities were canceled at Columbine in Littleton, Colorado.

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

"We are currently investigating what appears to be a credible threat possibly involving the schools," Jefferson County Sheriff's department said earlier in a tweet. "Children are safe. Deputies are at the schools. "

The FBI is leading the investigation, said Mike Taplin, a spokesman for the sheriff's department. The FBI did not respond to a request for comment, but the state's education department said in a tweet that the threat came from "an individual identified by the FBI. "

Two hours after announcing the lockouts, the school system reported that all its students and staff were safe, adding that students will be released from the schools and buses will 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 would also continue as scheduled - except at Columbine, where they're canceled for the day, the school said.

Since April 20, 1999, when two gunmen stormed Columbine, killed 13 people and wounded 24 more, 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.

"Getting threats is not out of the normal for Columbine High School," Taplin said.

The 20th anniversary of the shooting at Columbine will take place this Saturday. As the district prepares for the day's memorial events, it is fending off an onslaught of curious strangers who trespass in the parking lot of the high school - sometimes more than 30 people in a single day.

The 20th anniversary of the Columbine High School shooting is this Saturday. Students are pictured stopping to pay their respects at a makeshift memorial after the attack in 1999. Photo / AP
The 20th anniversary of the Columbine High School shooting is this Saturday. Students are pictured stopping to pay their respects at a makeshift memorial after the attack in 1999. Photo / AP

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 to be 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, another said there was a person with a gun outside of it. The threats were quickly investigated and found to be not credible.