What happened in the columbine shooting?

On April 20th, 1999, two disgruntled students - Dylan Klebold and Eric Harris - entered Columbine High School in Columbine, Colorado and began shooting in a discriminatory and non-discriminatory fashion at students and teachers (some people were targeted, others were not). They roamed the halls for over 30 minutes until the thrill was gone from killing. They then retreated to the cafeteria and committed suicide together.

After the fact, Klebold and Harris had shot and killed 12 students and 1 teacher, raising the death tally to 15 (including themselves). America was shocked that two 12th grade boys could commit such a horrendous mass killing, at a school of all places. Officials wanted to know "why?", and to this day, there are no real answers - only speculation and educated guesses. Most are satisfied with the notion that Klebold and Harris had been social outcasts during most of their high school lives, and may have had mental health issues. Others say that they had been bullied - especially by jocks. Jocks were targeted by the duo during the massacre, easily identified by their white caps.

We won't ever really know, though - the two didn't leave detailed notes about the massacre and kept to themselves.