Pais had "made some concerning comments in the past" and had an "infatuation" with the 1999 Columbine High School shooting and its perpetrators, Phillips said at the press conference.
Sol Pais flew from Miami to Denver on Monday and "immediately" bought a pump action shotgun and ammunition, FBI Denver Special Agent In Charge Dean Phillips told reporters Tuesday evening. Authorities requested the lockout in Jefferson County on Tuesday afternoon as they searched for the suspect, whose description was released on Twitter with a warning not to approach her. All after-school activities will go on as scheduled, except at Columbine, where they were cancelled as a precaution, the district said. Students will be released from schools normally; buses will be running on normal schedule.
Police identified the suspect as 18-year-old Sol Pais, whom they said travelled to Colorado from Miami and threatened the schools.
The Department of Public Safety urged schools to tighten security, and more than 20 put lockdowns in place.
Authorities said Pais arrived in Colorado last night and made threats to area schools.
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.
Then, at 12:20 p.m., Jefferson County Schools tweeted this: "We have placed #ColumbineHighSchool, #LeawoodElementary, @bhelementaryco and all mountain schools on lockout".
Columbine High, which is in Jefferson County, and several schools in the area were part of the lockout, the Jefferson County Sheriff's Office tweeted.
Pais was last spotted in the foothills of Jefferson County.
People visit the Columbine memorial after teens kicked off a voter registration rally, a day ahead of the 19th anniversary of the massacre at Columbine High School, in Littleton, Colorado, U.S., April 19, 2018.
The FBI is now looking for a woman they say is "obsessed" with the Columbine High shooting.
Jeffco Public Schools said the cancelations include all afternoon activities and athletic competitions.
Officials didn't provide further details or reveal where Pais traveled from, saying it would compromise their investigation.
Davis says "Troubled kids in our schools, and nobody knows about it. the teachers and administrators are the first line. they need to see that, and they need support and help".
Twenty years ago this month Columbine High School in Colorado became the scene of one of the worst schootings in history.
At the time, it was America's deadliest school shooting. There was no immediate indication whether Tuesday's lockdown has any connection to the anniversary.