It was on April 20, 1999 that two teenage gunmen killed 13 people in a shooting at Columbine High School in Littleton, Colorado.
At 11:20 a.m., Dylan Klebold and Eric Harris began shooting students outside the school, before moving inside.
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SWAT team officers entered the school at about 3:00 p.m., by that time - the two had killed 12 fellow students and a teacher - and had wounded another 23.
Around noon, the teens committed suicide.
In the days following the shootings, many speculated that Klebold and Harris targeted jocks, blacks and Christians, but later investigations would find that victims were chosen at random.
The original plan was for two bombs to explode in the cafeteria, forcing students outside - but when their homemade bombs didn't work, they carried their attack into the school.
Columbine reopened in the fall of 1999. The shootings stood as the worst school shooting in U.S. history until 2007, when 32 were killed at Virginia Tech University.