(CNN) Here is some background information about the deaths of 13 people at Columbine High School in Littleton, Colorado, on April 20, 1999.

Facts

Twelve students and one teacher were killed by students Dylan Klebold, 17, and Eric Harris, 18.

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 killed themselves with gunshot wounds to the head in the school's library at approximately 12:08 p.m. on the day of the shootings.