Authorities say there is no threat to Columbine High School but students will be kept inside for the rest of the school day after someone called in a bomb threat against the school.
A spokesman for the Jefferson County Sheriff's office, Mike Taplin, said that a person called Thursday morning, claiming to have placed explosive devices in the school and to be hiding outside with a gun. Taplin says nothing to validate the threat was found at the school, where 12 students and a teacher were killed by two students in 1999.
The sheriff's office says it is working to locate the caller. The threat drew a large police response but agencies began leaving shortly after midday.
Students at 23 other schools nearby were also temporarily placed on lockout, meaning their doors were locked but classes continued normally, as the threat was investigated.
Dozens of other threats were made across the country as well, and similarly appear to be a hoax.
The New York City Police Department said the threats sent Thursday were meant to cause disruption and compel recipients into sending money and are not considered credible.
Some of the emails had the subject line: "Think Twice."
The Palm Beach County, Florida, sheriff's office and the Boise, Idaho, police said they had no reason to believe that threats made to locations in those areas were credible.