In this image taken from video, a group of freed students and faculty run from Columbine High School in Littleton, Colo., after two students swept through their school with guns and explosives. (Hal Stoelzle/AP)
Thursday marks the 18th anniversary of one of the deadliest school shootings in U.S. history.
On April 20, 1999, Eric Harris and Dylan Klebold went on a shooting rampage at Columbine High School in Littleton, Co. Twelve students and one teacher were killed, and more than 20 others were wounded.
Harris and Klebold turned the guns on themselves after carrying out the attack.
More than 100 explosive devices were recovered from the scene.