A lockout at Columbine High School and nearby schools in Colorado ended at the end of the school day Thursday, and students went safely home, the Jefferson County sheriff's office said.
The alert had been issued earlier in the day after several telephone threats.
A lockout means that school business goes on as usual but that no one is allowed to enter or leave the building, sheriff's spokesman Mark Techmeyer told the Los Angeles Times.
"We've had several phone calls back-to-back threatening Columbine High School," Techmeyer said. "We've had hundreds of these since the massacre, and we take them all seriously. It is normally a short-lived event."
On April 20, 1999, Columbine High School was the scene of one of the nation's worst massacres. Two students killed 12 students and one teacher. They injured 24 other students, then committed suicide.
In addition to the high school, numerous elementary and middle schools in the area were also placed on lockout status as a precaution, Techmeyer said. Deputies were dispatched to the high school, he said.