LITTLETON, Colo. (AP) - Columbine High School, the scene of a school shooting that left 13 people dead in 1999, went on high security alert after receiving a series of threatening phone calls, law enforcement said Thursday.
    
The school received the first phone call at 10 a.m. Thursday and others came "over the next 2-3 minutes," said Jacki Kelley, a spokeswoman for the Jefferson County Sheriff's Office.
    
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.

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