Columbine High School massacre
On April 20, 1999, high school senior students Eric Harris and Dylan Klebold murdered 13 students and one teacher in a school shooting and attempted bombing at Columbine High School in Columbine, Colorado, United States. Twenty people were injured by the gunfire, and three others were injured while trying to escape.
Source: Wikipedia — Columbine High School massacre (CC BY-SA 4.0)