On 20 April 1999, two teenagers, Dylan Klebold and Eric Harris, walked into their school, Columbine High, with their trench coats loaded with guns, knives and explosives, with the intention of killing as many people as possible before committing suicide.
In the end, they killed 12 students and one teacher before killing themselves.
The Columbine High School massacre left the suburban Colorado community not only shocked and angry but utterly confused.
Nobody could figure out why these two apparently fairly typical teenagers from stable homes had committed such a horrific crime.
According to documentary evidence that the boys left behind, Klebold and Harris had been planning the attack, in which they hoped to kill as many as 500 of their peers, for more than a year.
Diaries created a confusing image of the two boys downloading recipes for explosives and making them, at the same time as worrying about who they should take to the prom.
The two boys had been friends for several years, and although they found it difficult to fit in at their high school, they were not otherwise abnormal.
The only clue that could provide any possible motive was a general sense of hatred that they both felt toward the world and toward people in general.
The objects of their hatred, however, were often contradictory: they hated both minorities and racists, for example.
Even after working on the case for a year, the lead investigator was unable to give any clear-cut answers: “There’s not an easy answer,” she said, “I can’t tell you why it happened.”