"She did make statements that were threatening to schools and she did purchase a firearm. and that's why she's a credible threat", said Patricia Billinger, a spokeswoman with the Colorado Department of Public Safety.
Authorities said they have not identified any connections she has with Colorado and most of her threats were spoken.
Police said the 18-year-old woman, Sol Pais, made threats that led to lockouts at many Colorado schools Tuesday, including Columbine High School.
Pais is about 5-foot-5, has brown hair and was last seen in the foothills of Jefferson County wearing a black T-shirt, camouflage trousers and black boots, authorities said.
In a tweet, the Jefferson County Sheriff's Office said, "We are now investigating what appears to be a credible threat possibly involving the schools".
On April 20,1999, two students, Dylan Klebold, 17, and Eric Harris, 18, killed 12 students and one teacher in a mass shooting at Columbine.
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Authorities said Pais is armed and considered extremely unsafe and should not be approached.
Federal Bureau of Investigation agents, along with Jefferson County Sheriff's deputies and Colorado state troopers, have searched for her, including hunting through rural Jefferson County foothills throughout the day and have published her name and photo in local media.
Columbine and dozens of other Denver-area schools were issued a security threat Tuesday afternoon and a decision will be made early Wednesday as to whether schools will be in session or other security measures taken, officials said in a late-night press conference. He told Hickenlooper he'd like to see more funding go to educators.
Aurora Public Schools, east of Denver, said all of its schools were "placed in secure perimeter". At the time it was the deadliest school shooting in U.S. history.
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.
On Tuesday, law enforcement said students would remain in school through the day.
Students leave Columbine High School on Tuesday.
All of the students are safe, the Jefferson County Sheriff's office said on Facebook.
School security chief John McDonald talks with judicial specialist Maryann Peratt in Columbine High Schools new library, which was built after the 1999 attack.