Columbine Massacre (USA)

The Columbine High School massacre occurred on 20 April 1999, at Colimbine High School in Columbine, Colorado. Two students, Eric Harris and Dylan Klebold, embarked on a massacre, killing 12 students and a teacher, as well as wounding 23 others, before committing suicide.

It is the fourth-deadliest school massacre in United States history, after the 1927 Bath School Disaster, 2007 Virginia Tech massacre and the 1966 University of Texas massacre, and the deadliest for an American High School.
The above still, Eric (left) and Dylan (right), is taken from the infamous Columbine High School cafeteria surveillance video.

School shootings were already a problem before 20 April 1999. Dylan Klebold and Eric Harris knew that theirs had to stand out. So they planned to make every previous incident look insignificant, and they videotaped their boast so the world would know what they had set out to do.
And so they turned Columbine High School into an abattoir: murdering 13, wounding 24 other people and then, finally, killing themselves in a drama seen live on television.
It was not quite the 250 they had hoped to kill, but it was enough to make the incident the worst school shooting in American history.

This sudden eruption of violence in the middle of one of the most solidly upper middle class communities in America set off months of soul searching. Parents and school officials discussed the prevalence of violent music and video games. Parents asked: what are the warning signs that our children are turning out to be their own enemies? On their tapes, Klebold and Harris talk about anger management but not the expected kind. Rather, they were learning to ratchet up their anger and yet keep it secret from everyone else -- until the day they had to turn it on full blast.