(CNN) - Saturday, April 20th marks the twentieth anniversary of the mass shooting at Columbine High School in Colorado.
On this day in 1999, twelve students and one teacher were killed.
17-year-old Dylan Klebold and 18-year-old Eric Harris carried out the massacre.
They made videos before the attack talking about their plans, and even apologizing to their parents for it.
Both died by suicide.
They were found in the school's library with gunshot wounds to the head.