DENVER: Authorities in Colorado placed Columbine High School and at least a dozen other surrounding public schools under a “lockout” security alert on Tuesday, citing the investigation of an unspecified “credible threat.”

The alert comes four days before the 20th anniversary of the massacre at Columbine when two heavily armed students at the suburban Denver high school fatally shot 12 classmates and a teacher before committing suicide.

Tuesday’s lockout, less serious than a public safety “lockdown,” means that activities inside the schools may continue as usual but entry and exit was restricted, the Jefferson County Sheriff’s Office said on Twitter.

Officers were “investigating what appears to be a credible threat possibly involving the schools,” the sheriff’s department said on Twitter, adding that students were safe and additional deputies were dispatched to the schools.

A sheriff’s office spokesperson, Mike Taplin, said the anniversary of the Columbine mass shooting was not a “direct” factor in the threat.
Security alerts and safety drills have become commonplace in public schools across the US in the years since the Columbine massacre as campus gun violence has increased.