Fifteen schools in the suburbs of Denver, including Columbine High School, were placed on lockdown Tuesday due to a "credible" threat of violence intercepted by local authorities.

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The Jefferson County Sheriff's Department tweeted Tuesday afternoon that the lockdown included Columbine as well as 14 other schools in the mountainous region around Colorado's capital.

The lockdown came just days before the 20th anniversary of the mass shooting at Columbine High School.

"We have placed #ColumbineHighSchool, #LeawoodElementary, @bhelementaryco and all mountain schools on lockout. No further information at this time," added a message from the county school system.

On April 20, 1999, 12 students and one teacher at Columbine were killed by two students who later killed themselves. The attack is now considered the 12th deadliest mass shooting in modern U.S. history.

Tuesday's lockdown also came on the 12th anniversary of the mass shooting at Virginia Tech, in which 32 people were killed.