On April 20, 1999, tragedy struck the town of Littleton, Colorado, when two teenagers went on a shooting spree at Columbine High School. The Columbine High School Massacre was, at the time, the worst school shooting in U.S. history.

The tragedy began at 11:10 in the morning, when 17-year-old Dylan Klebold and 18-year-old Eric Harris arrived at the school and, dressed in trenchcoats, placed two duffel bags containing homemade propane bombs in the school cafeteria and set them to go off at 11:17. Fortunately, the bombs failed to detonate. After realizing that the bombs didn’t go off, at 11:19, the pair initiated the second part of their plan and began shooting students. They started shooting outside the school and then moved inside to the library, where they found most of their victims. They killed 12 students, 1 teacher and wounded 20 other people before turning their guns on themselves.

Because the shooters committed suicide and left behind no explanation of their crime, no certain determination of their motive was ever made. Rumors ran wild, placing the blame for the shootings on video games, music, the Goth subculture, and more, but no evidence was ever found for any of these far-fetched ideas. It seems likely that revenge for being bullied at least had something to do with the shooting. In the wake of the tragedy, schools across the nation enacted zero-tolerance policies regarding threats of violence and disruptive behavior from students.