Timeline: Mass school shootings in U.S. history

Dylan Klebold and Eric Harris are shown in the Columbine High School cafeteria on the day they...

Dylan Klebold and Eric Harris are shown in the Columbine High School cafeteria on the day they killed twelve students and one teacher before killing themselves on April 20, 1999, in a still image from a security camera released by the Jefferson County Sherrif's departement in Golden, Colorado, December 13. (REUTERS FILE PHOTO)

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