FBI hunts woman over Columbine threat

A woman suspected of making threats against Columbine High School is being sought by the FBI, just days before the 20th anniversary of a mass shooting.

Students leave Columbine High School following a lockdown prompted by threats. (AAP)

A woman described as "infatuated" with the 1999 Columbine High School massacre is being sought by the FBI in Colorado while authorities issued a security alert to Columbine and other Denver-area schools, citing a threat of violence.

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

At the time it was the deadliest school shooting in US history.

On Tuesday, the Jefferson County Sheriff's Office placed Columbine and 21 surrounding schools under a security "lockout," allowing activities inside to continue as usual but restricting entry and exit at the schools.

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.

The sheriff's office said on Twitter that a woman identified as Sol Pais, 18, was being sought by the Federal Bureau of Investigation and sheriff's deputies in connection with the case. It said she was armed and "considered to be extremely dangerous."

A sheriff's spokesman, Deputy Mike Taplin, later told a news briefing that Pais, reported by the Denver Post to be a Florida resident, had travelled to Colorado on Monday night 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.

Authorities said that school security was beefed up and students were safe.

Taplin said that the FBI mounted the investigation and alerted local authorities in Colorado after determining Pais posed a "credible threat."

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

The Denver Post reported that one of its correspondents called the home of Pais' parents in Surfside, Florida, located near Miami, and that a man identifying himself as an FBI agent interrupted the call and said the agency was interviewing them.

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