She made "credible" - but unspecific - threats after traveling from Miami to Denver on Monday night, and is considered unsafe, said Dean Phillips, the special agent in charge of the local Federal Bureau of Investigation office. "She is armed and considered unsafe", said Mike Taplin with the Jefferson County Sheriff's Office, CBS Denver reported.
"This has become a massive manhunt", Phillips said in a Tuesday night press briefing.
"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 said, "we consider her to be a credible threat - certainly to the community and, potentially, to schools".
Students wearing backpacks came out the main door at Columbine at about 2.55pm local time, soon after school officials announced that students in all the schools put on lockdown were safe and would be dismissed at their normal times.
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. The state agency said individual districts would provide details about closures.
The Colorado Department of Public Safety later alerted schools throughout the Denver metropolitan area to the threat, recommending they conduct a "controlled release" of students from classes in the afternoon. Denver is adjacent to Jefferson County.
Johnson says "It just made me really sad that our kids have to do lockdown drills, and have to be talked to about these kind of things".
On Tuesday, law enforcement said students would remain in school through the day. "We want our schools to be a safe place for kids to learn".
Caution spread to schools throughout the Denver area, which also issued lockouts or said they were monitoring the situation. Officials said extra security would be present on the affected campuses.
Some schools released their students after additional security was called in and canceled evening activities or moved them inside. Students at high schools across the country are expected to walk out of classes Friday, the 19th anniversary of the Columbine shooting, in their latest push for gun control.
Columbine High in Littleton and several schools in the area were earlier placed on a "lockout", the Jefferson County Sheriff's Office tweeted.
The 20-year anniversary of the shooting is on Saturday.
He said he had no other information. Officials emphasise taking every threat - no matter how vague - seriously.
Then, at 12:20 p.m., Jefferson County Schools tweeted this: "We have placed #ColumbineHighSchool, #LeawoodElementary, @bhelementaryco and all mountain schools on lockout". "There may be some connection to Columbine but it wasn't a specific directed threat towards Columbine".
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