Twenty years ago, at a school in Littleton, Colorado, as students prepared for lunch, their lives were irrevocably changed in one of the most distressing school shootings America had ever seen. Now, it's not even amongst the ten worst mass shootings in modern American history.

Eric Harris and Dylan Klebold were twelfth grade students at Columbine high school. Armed with homemade bombs and guns they stormed their school, killing twelve fellow students and one teacher before turning the guns on themselves.

The USA went into mourning and for a time, Columbine became the most famous school in the world.

The attack sparked debate over gun culture and gun control. The influence of the internet, violence in video games, social isolation, the use of antidepressants by teenagers and even metal music were all called into question.

The the days after the massacre, as candles left in memorials burned down to the wick and flowers began to wilt, promises were made. Promises by politicians, state officials, school and community leaders that things would change. Some things did. Schools across the country introduced new security measures such as see-through backpacks, metal detectors, security guards and practice drills for students. The police also reassessed how they respond to school shootings after criticism over their slow response during the attack. They now enter buildings where they there is an active shooter, previously they set up a perimeter around the building before even thinking about moving on the suspect.