On April 20, 1999, Eric Harris and Dylan Klebold went on a shooting rampage at Columbine High School in Littleton, Colorado, killing 12 students and one teacher. More than 20 others were wounded.
The pair made home videos prior to the attack making references to what they were going to do and apologizing to their parents for it.
Harris and Klebold turned the guns on themselves after carrying out the attack.
More than 100 explosive devices were recovered from the scene.
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