Still no-one can understand why these two school friends went to school with a deadly arsenal, intent on slaying their classmates.
On 19 April 1999, Columbine was just a town in Colorado, and the name of the high school that served the local kids. But from the following day onwards, no-one would ever speak the name again without being reminded of the one of the worst school shootings in history.
Two friends, Eric Harris, 18, and Dylan Klebold, 17, both students at Columbine High, arrived at school that day with a devastating arsenal, which they used to kill 12 of their classmates and one teacher.
Before the cafeteria at Columbine filled with students on their lunch break, Harris and Klebold planted two large homemade bombs and set them to explode at 11.17am, when the room would be at full capacity.
Then they retreated to the car park, their guns pointed ready to pick off any students escaping from their blast. The bombs failed to explode.
Yet Harris and Klebold still managed to create carnage that day. Stalking the school corridors and library, they fired at random. As well as the 13 innocent lives they took, they shot 21 more, leaving some with injuries that paralysed them for life.
By 12.06pm, both killers were also dead, having shot themselves in the head. The Columbine shooting had lasted just under one hour from start to finish. The murders had happened in just 16 minutes.
In the years since Columbine many motives have been suggested.
Some say violent films or videogames drove Harris and Klebold to murder. Others say they were bullied and killed in revenge. No-one can say for sure what caused Harris and Klebold to take so many lives that terrible day of the Columbine shooting.
But the question keeps being asked as people search to understand why such young men could have so much hatred in their hearts.