Classes are canceled at Columbine High School to mark the 16th anniversary of the shootings that killed twelve students and a teacher.

A moment of silence to honor those lost on April 20, 1999 will be held at 11:20 a.m. in the school's library, which was rebuilt following the attack.

The tradition of canceling classes on the shooting's anniversary started under the principal at the time, Frank DeAngelis. He retired last year and the school's new leader, K.C. Somers, sent a letter to students earlier this month saying the tradition would continue for the foreseeable future.