Columbine High School has faced a security alert due to an unspecified threat to the establishment, the scene of a 1999 massacre, officials said.
Monday's alert or lockout - during which lessons carried on as normal, but with the perimeter secured due to a threat from outside the building - was lifted after about three hours.
Officials did not specify the nature of the threat, which triggered the lockout shortly after classes began in the morning.
It was lifted around lunchtime.
The school regularly gets false alarm threats, officials told local media.
Students Eric Harris and Dylan Klebold attacked Columbine High School in Colorado on April 20, 1999, killing 13 people and wounding 23 before turning their guns on themselves.
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