Shortly after 11 a.m. on April 20, 1999, 12th-graders Dylan Klebold and Eric Harris arrived at their school - Columbine High in the southern suburbs of Denver - in separate cars.
They walked into the school cafeteria, where they placed two duffel bags, each containing a home-made 20-pound propane bomb set to explode at 11:17 a.m. - in the middle of the school's busiest lunch period.
They then walked back to their cars. They boys had planned to use weapons hidden beneath their black trench coats to fire on students fleeing the school after the blast. When the bombs didn't go off, the boys opened fire on students outside the school and then walked back inside, shooting other students along the way.
The boys made their way to the school library where 52 students, two teachers and two librarians were taking shelter. There, Klebold and Harris went from hiding spot to hiding spot shooting some students and sparing others. In particular, they targeted students of color and students who played sports at school.
They roamed up and down a few hallways, tossing pipe bombs and firing at random. Realizing police had arrived outside the school, Klebold and Harris returned to the library and, on the count of three, shot themselves.
By 12:08 p.m., the incident was over. Twelve students and a teacher were dead and 21 others were wounded.
Investigators would learn the two boys had been fascinated by Goth culture and with violent shoot-'em-up style video games. The previous December, Harris had made a video for an economics class in which he and Klebold called themselves "Hitmen for Hire" and declared themselves members of "the Trench Coat Mafia".
The two had felt ostracized by other students at Columbine High and had planned the bombing to retaliate for being bullied.
Among the items Klebold and Harris left behind included videos of themselves discussing their plans for the massacre and discussing where would be the best place to plant their bombs. Harris had even left a cassette tape on his kitchen table in which he explained his plans.