LITTLETON, Colo. (AP) - Columbine High School and about a dozen other schools are on lockdown as law enforcement says it is investigating what appears to be a credible threat possibly involving the schools.

School officials say the doors were locked but classes were continuing Tuesday at Columbine and two other nearby schools and at other schools farther away in the district.

The Jefferson County Sheriff's Office said deputies were posted at the schools involved.

The lockdown comes just days before the 20th anniversary of a mass shooting at the school that killed 12 students and a teacher. There was no immediate indication whether Tuesday's lockdown has any connection to the anniversary.

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