Denver-area public schools were closed on Wednesday as authorities searched for a young Florida woman who flew to the city and bought a gun after becoming "infatuated" with the mass shooting at Columbine High School.

The FBI said Sol Pais, 18, was "considered to be extremely dangerous" and "made threats to commit an act of violence in the Denver metropolitan area" just days before the 20th anniversary of the attack that killed 13 people.

All schools in the Denver area were urged to tighten security because the threat was deemed "credible and general," said Patricia Billinger, a spokeswoman for the Colorado Department of Public Safety. Columbine and more than 20 other schools outside Denver locked their doors for nearly three hours on Tuesday afternoon before Wednesday's complete closures were announced.

The Jefferson County Sheriff's Office and the FBI say Ms Pais travelled to Colorado from Miami on Monday night and bought a pump-action shotgun and ammunition.

Denver Public Schools said that all facilities and programmes would be closed Wednesday, and there would 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 due to the ongoing safety concern.

On Tuesday, some schools released their students after additional security was called in and canceled evening activities or moved them inside.