ON THIS DAY: Mass shooting at Columbine HS

LITTLETON, Colorado -- April 20 marks the anniversary of one of the deadliest school shootings in U.S. history.

RELATED: Columbine shooter's mother talks exclusively to ABC

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.

Click here for more stories, photos, and video on the Columbine school shooting.