Colorado authorities are searching for an armed woman accused of contributing to a threat that caused more than 20 schools, including Columbine High School, to be placed on lockout.

Police say the woman, identified as 18-year-old Sol Pais, is “infatuated” with the 1999 Columbine High School massacre.

The alert for her came four days before the 20th anniversary of the Columbine tragedy, in which two heavily armed teen boys stormed their high school and fatally shot 12 classmates and a teacher before committing suicide

Jefferson County Sheriff’s Office spokesman Deputy Mike Taplin later told a news briefing that Ms Pais, reportedly a resident of Florida, had travelled to Colorado on Monday and “made threats to commit an act of violence in this area.”

“I believe it to be a general threat to schools, not a specific school,” he said.

Police said Ms Pais was last seen in the foothills west of Denver. She was considered extremely dangerous and should not be approached.

Authorities said that school security was beefed up and students were safe. Less serious than a lockdown, a lockout means that classes continue as normal inside the schools, but entry and exit is restricted.

Officer Taplin said the FBI had mounted the investigation and alerted local authorities in Colorado after determining Ms Pais posed a “credible threat.”

We are currently investigating what appears to be a credible threat possibly involving the schools. Children are safe. Deputies are at the schools. Multiple schools are on lockout in Columbine area & Mountains. More details to come #JeffCo pic.twitter.com/LwS6VJKxdV

— Jeffco Sheriff (@jeffcosheriffco) April 16, 2019

An FBI bulletin described Ms Pais as “infatuated with (the) Columbine school shooting”, saying she was believed to be in the Denver-Littleton area “attempting to buy firearms”.

Jefferson County Public Schools tweeted a list of 22 elementary, middle and high schools placed under lockout.

School officials said the doors were locked but classes continued at Columbine High School and two other nearby schools, as well as at other schools farther away.

-with AAP