LITTLETON, Colo. (AP) -- A lockout has been lifted at Columbine High School in Colorado after law enforcement said it received a series of threatening phone calls.
Jefferson County Sheriff's Office spokeswoman Jacki Kelley said the school received the first phone call at 10 a.m. Thursday and others came "over the next 2-3 minutes."
The school was placed on lockout, which prevents people from entering or leaving but allows them to freely circulate inside. The alert also applied to a half-dozen other schools in the area and was issued shortly before noon. Shortly after 2 p.m. the lockout was lifted.
Kelley said the department has sent "additional resources" to protect the school and is investigating the threats. She would not detail the threats.
In 1999, two students opened fire at Columbine, killing 12 classmates and a teacher and wounding 26 others before killing themselves in the school's library.