'Credible threat' sends Columbine and 20 other schools into lockdown as 20th anniversary of shooting nears

Nearly two dozen schools in Colorado, including Columbine High School, are reportedly on "lockdown."

According to KOAA, officials said that a "credible threat possibly involving the schools” had caused the lockdown.

A tweet sent out by the school system said that schools are conducting "business as usual" inside the buildings.

The lockdown comes just days before the 20th anniversary of the Columbine shooting, which occurred April 20, 1999.