Columbine High School, the site of one of the US's deadliest school shootings eight years ago, was evacuated after several bomb threats, officials say.
Students were moved to a nearby school where their parents were able to pick them up, a police spokesman said.
There were several hundred pupils in the Colorado school at the time.
A bomb squad and police dogs searched the building but officials said there was no evidence at this point to support the threat.
More than one bomb threat was made by telephone to officials on Thursday morning but it is unclear whether any reference to the 1999 attack was made by the caller.
"Most of the threats that have been associated to that school made reference to past events that occurred there," said Jefferson County Sheriff's spokeswoman Jacki Kelley.
"But a bomb threat to a school is as serious at any school as it is at Columbine."
In 1999, two pupils at the school shot dead 12 fellow students and a teacher before killing themselves.