The Columbine lockout is under investigation for a "credible threat"

All schools in the Denver area are expected to undergo a "lockout" and controlled release, according to a subsequent tweet from the Colorado Department of Education.

This comes after the FBI has identified an individual who has posed a credible threat to schools, the tweet says.

Jefferson County School officials tweeted that all students and staff were safe and that students would be released on a normal schedule. "We will have additional security staff on site in all affected schools," said Jefferson County Public Schools.

Columbine High in Littleton and several schools in the area had already been placed on a "lockout," tweeted the Jefferson County Sheriff's Office. The threats were not targeted at a particular school, according to Sheriff spokesman Mike Taplin.

Darran Simon from CNN contributed to this report.