(CNN) - Columbine High School, site of a 1999 massacre by two students, was shut down after receiving bomb threats on Thursday, said the Jefferson County Sheriff's Office near Denver.
The calls were made about 9:20 a.m. claiming multiple bombs were inside and the caller was waiting outside the school with a gun.
Authorities conducted a search, found no suspects, and decided the threat was baseless about 12:45 p.m., the sheriff's office said.
County school officials placed 22 other schools on lockdown as a precaution.
On April 20, 1999, Eric Harris, 18, and Dylan Klebold, 17, shot dead 12 fellow students and one teacher at the school in Littleton, Colorado. The pair then killed themselves in the school library.
Authorities did not link Thursday's threat at Columbine to dozens of institutions across the United States and Canada receiving emailed bomb threats, causing evacuations and searches of buildings.
Friday is the sixth anniversary of the Sandy Hook killings, when Adam Lanza, 20, killed his mother and then gunned down 20 children, ages 6 and 7, and six adults at a school in Newtown, Connecticut, before killing himself.