LITTLETON, Colo. (AP) - All schools in the Denver area are being urged to tighten security after what appears to be a credible threat caused a lockout at Columbine High School and more than 20 other schools in the area.
 
Patricia Billinger, a spokeswoman for the Colorado Department of Public Safety, says the agency made the recommendation because Tuesday's threat was deemed "credible and general."
 
At least one school district heeded the advice. Aurora Public Schools tweeted that students were being released in a "controlled manner," and additional security was put in place throughout the district as a precaution.
 
The lockouts come just days before the 20th anniversary of a mass shooting at Columbine High School that killed 12 students and a teacher. Authorities say they are looking for a woman suspected of making threats. They say she should be considered armed and extremely dangerous.