At approximately 11:19 a.m. on April 20, 1999, Klebold, 17, and Harris, 18 opened fire inside Columbine High School, slaughtering 12 students and one teacher, and wounding 23 others before killing themselves.
Wearing black trench coats, the two murdered 10 students in the library, where the pair later committed suicide.
The motives behind their massacre continue to be debated to this day. Harris and Klebold were initially reported as goths and loners who were part of the "Trench Coat Mafia." Other accounts indicated they were bullied by jocks and sought revenge on classmates who treated them as social outcasts. However, note experts, none of those theories are true.
In reality, the two were eventually deemed to be merely troubled and depressed psychopaths who dreamed of becoming domestic terrorists as famous as Oklahoma City bomber Timothy McVeigh.
Scroll through this RadarOnline.com gallery to learn more about that fateful day.
On Tuesday morning, April 20, 1999, Harris and Klebold arrived separately at CHS shortly after 11 a.m.
They placed two duffel bags in the cafeteria, each packed with propane bombs. The devices were set to detonate at 11:17 a.m., during the "A" lunch shift, when nearly 500 students would gather to eat. However, the cafeteria bombs failed to detonate.
At 11:19 a.m., witnesses reported hearing the words "Go! Go!" before Klebold and Harris pulled their guns from beneath their trench coats and began shooting students outside the school’s west entrance.
Rachel Scott, 17, was the first child killed, when she was hit four times with rounds fired from Harris's gun; one shot was to her left temple.
More Shots Fired
The two made their way through the school, spraying bullets as the walked. Witnesses heard one of them say: “This is what we always wanted to do. This is awesome!”
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Heroes in Action
As the rampage continued outside the school, business teacher and coach William "Dave" Sanders ran to the cafeteria and sounded the alarm. He, along with two of the school's janitors, helped get more than 100 students out of the path of danger by herding them away from the shooters.
Shot From Behind
Sanders was trying to get students safely hidden in classrooms when Harris shot him from behind. The business teacher was hit in the torso, head and neck, and he bled to death waiting for help.
At 11:29 p.m., the gunmen entered the library, where as many as 52 students, two teachers and two librarians were hiding. Harris and Klebold killed 10 and injured 12 more before walking out of the room at 11:36 – just seven minutes later.
Trail Of Terror
Over the next half hour, the two continued their trail of terror, shooting at random objects in the school. Most of the students had escaped and evacuated by this point. The two re-entered the library shortly after noon and shot through the west windows at police, who returned fire.
By just after noon, both gunmen had killed themselves. Reports indicate that Harris fired his shotgun through the roof of his mouth, while Klebold shot himself in the left temple.
Just the Start
Authorities later revealed diaries indicated the two teenagers planned even more destruction, within the town of Columbine and beyond.