On this day in 1999, 13 people were killed at Columbine High School by two teenage gunmen, Dylan Klebold and Eric Harris.

The shooting began at about 11:20 a.m. Dressed in long trench coats, the two gunmen started firing at students outside of the building, then moved inside to continue shooting. By the time SWAT officers came to the school, 12 students and one teacher were already dead, as well as 23 wounded. Then around noon, the gunmen shot themselves, commiting suicide.

The massacre caught the nation’s attention quickly. In the days immediately following the shootings, many claimed that Klebold and Harris purposely chose jocks, African Americans, and Christians as their victims. One of the students, Cassie Bernall, was asked by one of the gunmen if she believed in God. Her response led to her being shot to death. That response was “Yes.”

Commentators railed against the so-called “Trench Coat Mafia” and “Goths,” and questioned why these groups and cliques were not monitored more closely. However, further investigation revealed that Klebold and Harris were not part of either group.

Columbine High School was closed for months, then reopened in the fall of 1999. The massacre left a wound on the Littleton community. A senior at the school was arrested after he threatened to “finish the job.” Later, a carpenter from Chicago placed 15 crosses in a local park on behalf of everyone who died on April 20. The parents of the victims tore down the two gravestones that said “In memory of Klebold and Harris.”

To show the world “that life goes on,” the Columbine school board voted to replace the library where students were shot with an atrium. The shootings at Columbine stood as the worst high school shooting in U.S. history until February 14, 2018 at 12:18 p.m. when 17 people were shot and many others wounded by a student gunman at Stoneman Douglas High School in Parkland, Florida.