The massacre of Columbine High School in Colorado is still considered one of the worst high school shootings in US history. On April 20, 1999, two high school students entered the school grounds armed with guns and homemade bombs.

Historians and researchers believe the attack on the school was indirectly related to the bombings of Oklahoma City and Waco, Texas. Journals and videos made by Harris and Klebold stated the hoped to create a tragedy that would outdo the bombing of Oklahoma City. It has also been found that Harris and Klebold had hoped the original date to be April 19, but when their homemade bombs were not ready, they had to push the date back to the 20th.