Security fears have soared because Saturday is the 20th anniversary of what at the time was the deadliest school shooting in U.S. history, setting off an era of mass shootings in USA schools and other public venues.
The FBI described her as "infatuated" with the Columbine attack.
Surfside Police Sgt. Marian Cruz confirmed that parents of 18-year-old Sol Pais last saw her on Sunday and reported her missing on Monday.
The FBI's Denver office and the Jefferson County Sheriff's Office are searching for a woman who allegedly made threats against schools in the Denver metro area Tuesday. She had been last seen in a black T-shirt, camouflage trousers and black boots.
The FBI's Rocky Mountain Safe Streets Task Force issued a notice Tuesday for police to be on the lookout for 18-year-old Sol Pais.
Pais is believed to be armed and extremely unsafe.
All schools in the Denver area were urged to tighten security, and some canceled evening activities or moved them inside. "As far as this case goes, I want to stress that, at this time, there have been no threats to M-DCPS schools".
Police in the USA state of Colorado are searching for an 18-year-old woman suspected of making threats that led to a lockout at several Denver-area schools.
"We're notifying all staff and parents that the lockouts have been lifted and our operations will resume as normal", Dan Dougherty, the school district's communications chief, said late Wednesday morning.
All classes and extracurricular activities for about a half-million students were canceled as a precaution.
Dean Phillips, the special agent in charge of the FBI's Denver office, said Pais had made comments in the past that expressed "an infatuation" with the Columbine massacre and its perpetrators. "There may be some connection to Columbine but it wasn't a specific directed threat towards Columbine".
A message left for an Federal Bureau of Investigation spokeswoman wasn't immediately returned. Law enforcement requested the lockout after a "suspicious person" was seen in the area, says a rep for JeffCo Public schools. Fourteen other school districts in the area were closed as well, according to Colorado Public Radio.
The lockdown comes just days before the 20th anniversary of a mass shooting at the school that killed 12 students and a teacher.