Columbine and other Colorado schools placed under security alert

By Keith Coffman

DENVER (Reuters) - Authorities in Colorado placed Columbine High School and more than 20 surrounding public schools under a "lockout" security alert on Tuesday, citing the investigation of an unspecified "credible threat."

The alert came four days before the 20th anniversary of the April 1999 massacre at Columbine, when two heavily armed students stormed the suburban Denver high school and 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 is 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.

The nature and circumstances of the threat were not immediately disclosed. But a sheriff's office spokesman, Mike Taplin, said the anniversary of the Columbine mass shooting was not a "direct" factor.

Jefferson County Public Schools tweeted a list of 22 elementary, middle and high schools placed under lockout, which was to remain in effect until normal release time. All after-school activities, including sports and practices, will go on as scheduled, except at Columbine, where they were cancelled as a precaution, the district said.

Security alerts and safety drills have become commonplace in public schools across the United States in the years since the Columbine shooting as campus gun violence has grown more frequent. In Denver public schools alone, there have been 22 lockdowns and 294 lockouts over the past two academic years, according to school data cited by the Denver Post.

(Reporting by Keith Coffman in Denver; writing by Steve Gorman; Editing by Leslie Adler and Bill Berkrot)