On April 20, 1999, two students stormed the suburban Denver school armed with shotguns and semi-automatic weapons, killing 12 students and a teacher before dying by suicide.

The woman accused of making threats to "commit an act of violence" in lead up to the 20th anniversary of the Columbine school shooting was found dead during a manhunt, according to local media reports.

The Colorado "lockout" is less serious than a "lockdown."