On April 20, 1999, two teenage gunmen dressed in black trench coats went on a killing rampage at Columbine High School in suburban Denver.

They shot and killed 12 classmates and a teacher and wounded two dozen others in what was then the United States’ deadliest school shooting. The boys then took their own lives.

The shooting shocked the country as it played out on TV news shows from coast to coast. Images from the scene showed terrified students fleeing the school, SWAT officers waiting to enter and an injured boy trying to escape through a window.

FILE - In this April 20, 1999, file photo, women head to a library near Columbine High School where students and faculty members were evacuated after two gunmen went on a shooting rampage in the school in the Denver suburb of Littleton, Colo. (AP Photo/Kevin Higley, File)
FILE – In this April 20, 1999, file photo, women head to a library near Columbine High School where students and faculty members were evacuated after two gunmen went on a shooting rampage in the school in the Denver suburb of Littleton, Colo. (AP Photo/Kevin Higley, File)
FILE - In this April 20, 1999, file photo, members of a police SWAT team march to Columbine High School in Littleton, Colo. (AP Photo/Ed Andrieski, File)
FILE – In this April 20, 1999, file photo, members of a police SWAT team march to Columbine High School in Littleton, Colo. (AP Photo/Ed Andrieski, File)
FILE - In this April 22, 1999, file photo, Columbine High School students, from left, Darcy Craig, Molly Byrne and Emily Dubin stop to pay their respects at a makeshift memorial set up in a park near the high school in Littleton, Colo. (AP Photo/Eric Gay, File)
FILE – In this April 22, 1999, file photo, Columbine High School students, from left, Darcy Craig, Molly Byrne and Emily Dubin stop to pay their respects at a makeshift memorial set up in a park near the high school in Littleton, Colo. (AP Photo/Eric Gay, File)
FILE - In this April 21, 1999, file photo, from left, Rachel Ruth, Rhianna Cheek and Mandi Annibel, all 16-year-old sophomores at Heritage High School in Littleton, Colo. (AP Photo/Laura Rauch, File)
FILE – In this April 21, 1999, file photo, from left, Rachel Ruth, Rhianna Cheek and Mandi Annibel, all 16-year-old sophomores at Heritage High School in Littleton, Colo. (AP Photo/Laura Rauch, File)
FILE - This April 21, 1999, file photo, shows the news media compound near Columbine High School in Littleton, Colo. (AP Photo/Ed Andrieski, File)
FILE – This April 21, 1999, file photo, shows the news media compound near Columbine High School in Littleton, Colo. (AP Photo/Ed Andrieski, File)
FILE - In this April 25, 1999, file photo, a young girl holds a rose as she is held during a prayer at a memorial service on for the victims of the Columbine High School shooting rampage in Littleton, Colo. (AP Photo/Eric Gay, File)
FILE – In this April 25, 1999, file photo, a young girl holds a rose as she is held during a prayer at a memorial service on for the victims of the Columbine High School shooting rampage in Littleton, Colo. (AP Photo/Eric Gay, File)
FILE - In this April 24, 1999, file photo, the casket bearing Columbine High School shooting victim Rachel Joy Scott is signed with notes of remembrance from family members as it sits at the Trinity Christian Center in Littleton, Colo. (AP Photo/Rick Wilking, Pool, File)
FILE – In this April 24, 1999, file photo, the casket bearing Columbine High School shooting victim Rachel Joy Scott is signed with notes of remembrance from family members as it sits at the Trinity Christian Center in Littleton, Colo. (AP Photo/Rick Wilking, Pool, File)
FILE - In this April 27, 1999, file photo, Krista Sleeth, Bev Fleer and Cindy Sleeth embrace while standing on a hill above Columbine High School in Littleton, Colo. (AP Photo/Michael S. Green, File)
FILE – In this April 27, 1999, file photo, Krista Sleeth, Bev Fleer and Cindy Sleeth embrace while standing on a hill above Columbine High School in Littleton, Colo. (AP Photo/Michael S. Green, File)
FILE - In this April 24, 1999, file photo, students embrace each other at a makeshift memorial for their slain classmates at Columbine High School on a hilltop overlooking the school in Littleton, Colo. (AP Photo/Bebeto Matthews, File)
FILE – In this April 24, 1999, file photo, students embrace each other at a makeshift memorial for their slain classmates at Columbine High School on a hilltop overlooking the school in Littleton, Colo. (AP Photo/Bebeto Matthews, File)
FILE - In this June 15, 1999, file photo, tape marks the line in front of the doors to the library in Columbine High School as members of the media took their first trip through the school in the southwest Denver suburb of Littleton, Colo. (AP Photo/David Zalubowski, File)
FILE – In this June 15, 1999, file photo, tape marks the line in front of the doors to the library in Columbine High School as members of the media took their first trip through the school in the southwest Denver suburb of Littleton, Colo. (AP Photo/David Zalubowski, File)
FILE - In this April 20, 1999, file photo, Fran Allison, right, comforts her daughter Brooke after they were reunited after a shooting at Columbine High School in Denver. (AP Photo/Ed Andrieski, File)
FILE – In this April 20, 1999, file photo, Fran Allison, right, comforts her daughter Brooke after they were reunited after a shooting at Columbine High School in Denver. (AP Photo/Ed Andrieski, File)
FILE - In this April 28, 1999, file photo, a woman looks at crosses posted on a hill above Columbine High School in Littleton, Colo. (AP Photo/Eric Gay, File)
FILE – In this April 28, 1999, file photo, a woman looks at crosses posted on a hill above Columbine High School in Littleton, Colo. (AP Photo/Eric Gay, File)
FILE - In this April 24, 1999, file photo, a boy looks through the fence at the Columbine High School tennis courts in Littleton, Colo. (AP Photo/Eric Gay, File)
FILE – In this April 24, 1999, file photo, a boy looks through the fence at the Columbine High School tennis courts in Littleton, Colo. (AP Photo/Eric Gay, File)
FILE - In this April 20, 1999, file photo, SWAT members run down Pierce Street while a Jefferson County, Colo. (AP Photo/David Zalubowski, File)
FILE – In this April 20, 1999, file photo, SWAT members run down Pierce Street while a Jefferson County, Colo. (AP Photo/David Zalubowski, File)

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