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LITTLETON, Colo. (AP) - The Jefferson County Sheriff's Office says Columbine High School, the scene of a school shooting that left 15 people dead in 1999, is on a high security alert after an undisclosed threat.

The sheriff's office says in a tweet that the school is on lockout, which does not require students to take shelter, after authorities were notified of a telephone threat. The alert also applies to a half-dozen other schools in the area and was issued shortly before noon.

In 1999, two students opened fire at Columbine High School in Littleton, Colo., killing 12 classmates and a teacher and wounding 26 others before killing themselves in the school's library.

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