VIDEO: 20 Years Later, survivors and families remember Columbine

On April 20, 1999, two armed students opened fire on faculty and classmates at Columbine High School in the Denver suburb of Littleton. Thirteen people – 12 students and one teacher – were killed during the massacre.

At about 11:20 a.m., principal Frank DeAngelis was alerted to students with bombs by his secretary. He ran into the hallway thinking that it was a senior prank. The horror struck immediately as bullets flew and glass broke. His worst nightmare. Since that day, there have been more than 200 school shootings in the United States – four in Colorado alone.

Read more: The Denver Post reports on the 20th anniversary of the shooting that left 13 dead on April 20, 1999