The Jefferson County Sherriff's Office today confirmed an 18-year-old woman named Sol Pais travelled to Colorado and made threats that sparked the lockdown.
Denver Public Schools said all facilities and programs will be closed Wednesday, and there will be no afternoon activities or athletic competitions.
The district said the decision to close campuses was in collaboration with other Denver metro-area school districts. "We want our schools to be a safe place for kids to learn".
Doors were locked at Columbine and at least over 20 other schools in the Denver area as the sheriff's office said it was investigating a possible threat against schools that was related to an FBI investigation.
"Pais, whom the JCSO said was armed and "considered to be extremely risky", was described as a white female, 5'5" tall, with brown hair. Columbine High School is in Jefferson County.
"She has made some concerning comments in the past. and because of that we were concerned", he said. The situation arose days ahead of the 20th anniversary of the deadly shooting that occurred there on April 20, 1999.
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.
An officer from Jefferson County, Colo., Schools listens on his radio as students leave Columbine High School on Tuesday. Authorities say they are looking for a woman suspected of making threats. The students walked to crosswalks and buses parked near the entrance.
Among them was Columbine High School, school officials said on Twitter.
Almost 20 years ago, two students, Dylan Klebold, 17, and Eric Harris, 18, killed 12 students and one teacher in a mass shooting at Columbine. "I know that this opens a wound, especially on an anniversary week, for those families that were most deeply impacted by this".
Ten minutes later, the district followed with more information: "To clarify, mountain schools include Evergreen High, Evergreen Middle, Conifer High, West Jeff Middle, West Jeff Elementary, Bergens, Wilmot Elementary, Marshdale Elementary, Elk Creek Elementary, Ralston Elementary, Rocky Mountain Academy, Parmalee Elementary".
In the years since the massacre, the shooting has become a grim obsession for people known as "Columbiners".
The district has also seen an increase in threats and concerning messages, which often come in the form of emails to the school or phone calls to the 24-hour dispatch center run by the district's security team. "We will have extra safety and security staff on site at all schools affected". The threats were quickly investigated and found to be not credible.