Almost 20 schools in Jefferson County went into 'lockout mode, ' meaning exterior doors are locked, but inside, operations and classes continue as normal.
Two teenage gunmen attacked Columbine on April 20, 1999, killing 12 classmates and a teacher.
The Denver FBI office was alerted to the potential threat early Tuesday by Miami FBI agents. A man who answered the door identified himself as her father and said he lost contact with Pais the night before.
Pais was last seen in the foothills west of Denver, was considered armed and extremely unsafe and should not be approached. Pais is thought to have flown from her home in Miami to Denver on April 16.
Jefferson County school officials tweeted that all students and staff were safe and students would be released on a normal schedule.
Denver area schools are closed as authorities search for a young Florida woman allegedly obsessed with the 1999 Columbine school massacre who traveled to Colorado and bought a gun days before the 20th anniversary of the school shooting.
"So we're in the middle of an interview, so we're just kind of dealing with the family right now", he said. It said there is "more information to follow shortly".
"Due to a credible threat to schools by an individual identified by the Federal Bureau of Investigation, the Department of Public Safety recommends all schools in the Denver metro area conduct a lockout and controlled release immediately", the Colorado Department of Education tweeted.
The timing of Pais' threats held significance for authorities. He did not elaborate on what she said.More news: Chelsea striker Giroud: I have to meet Marina
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Pais was "no longer a threat to the community", Patricia Billinger, a spokeswoman for Colorado's Public Safety Department told Reuters.
At least one school district heeded the advice.
Miami-Dade County Public Schools Superintendent Alberto Carvalho showed up to the school Wednesday to speak with students and announced that mental health counselors and the district's crisis team would be deployed at the school.
"We have been trying to find her ever since", Phillips told a news conference late Tuesday in Denver. Because of her travel here to the state. They called her "extremely risky".
Local schools were notified of the risk and around a dozen Denver area districts called off classes for the day. This means that the 1,700 students at Columbine are accustomed to lockouts. It was the deadliest high school massacre in US history, prompting school security changes across the country.
The students walked to crosswalks and buses parked near the entrance.
Jefferson County Sheriff's Department tweeted about the lockdown mid-afternoon Tuesday. There was no immediate indication whether there was any connection to the anniversary.
Colorado Gov. Jared Polis said federal, state and local law enforcement were "dedicating all of their resources to locate this unsafe individual".