This Day in History:, Columbine High School Massacre.br April 20th, 1999.br At 11:19 A.M., Dylan Klebold, 18, and br Eric Harris, 17, walked into their high school br and began opening fire on students and teachers.br Less than 20 minutes later, Klebold br and Harris had killed 12 of their classmates, br one teacher and had wounded 23 other people.br Less than an hour after they had br begun their rampage, the two died by br suicide after turning their guns on themselves.br The massacre in the Denver suburb br Littleton, CO, shook the nation and br made headlines around the world.br It sparked speculation on the teens' motive br and intensified debate on the contentious issue br of gun control that would continue for decades.