Columbine High School massacre

A school shooting and attempted bombing occurred on April 20, 1999, at Columbine High School in Columbine, Colorado, United States. The perpetrators, twelfth-grade students Eric Harris and Dylan Klebold, murdered 13 students and one teacher; ten were killed in the school library, where Harris and Klebold subsequently died by suicide. Twenty additional people were injured by gunshots and gunfire was exchanged several times with law enforcement with neither side being struck. Another three people were injured trying to escape. The Columbine massacre was the deadliest mass shooting at a K-12 school in US history until December 2012. It is still considered one of the most infamous massacres in the US for inspiring many other school shootings and bombings; the word "Columbine" has since become a byword for modern school shootings. As of 2025, Columbine is still both the deadliest mass shooting and school shooting in Colorado, and one of the deadliest mass shootings in the United States.

Harris and Klebold, who planned for roughly a year, and hoped to have many victims, intended for the attack to primarily be a bombing and only secondarily a shooting. The pair launched a shooting attack after the several homemade bombs they planted in the school failed to detonate. Their motive remains inconclusive. The police were slow to enter the school and were heavily criticized for not intervening during the shooting. The incident resulted in the introduction of the immediate action rapid deployment (IARD) tactic, which is used in active-shooter situations, and an increased emphasis on school security with zero-tolerance policies. The violence sparked debates over American gun culture and gun control laws, high school cliques, subcultures (e.g. goths), outcasts, and school bullying, as well as teenage use of pharmaceutical antidepressants, the Internet, and violence in video games and film.

Many makeshift memorials were created after the massacre, including ones using victims Rachel Scott's car and John Tomlin's truck. Fifteen crosses for the victims and the shooters were erected on top of a hill in Clement Park. The crosses for Harris and Klebold were later removed following controversy. Planning for a permanent memorial began in June 1999, and the resulting Columbine Memorial opened to the public in September 2007.

The shooting has inspired more than 70 copycat attacks (as of June 2025), dubbed the Columbine effect, including many deadlier shootings across the world.