An emergency vehicle is parked outside the Abundant Life Christian School in Madison, Wis., following a shooting on Dec. 16. Morry Gash/AP hide caption
Columbine High School
Columbine Principal Frank DeAngelis listens to his students, survivors of the shooting, in April 2019 as he attends "Columbine 20 Years Later: A Faith-based Remembrance Service" at Waterstone Community Church in Littleton, Colo. A dozen students and one teacher were massacred by two heavily armed students. Joe Amon/AFP via Getty Images hide caption
A small, growing group of survivors advises school leaders after mass shootings
At StoryCorps in Littleton, Colo., last month, siblings Lauren Cartaya and Zach Cartaya said they continue to cope with the trauma of the 1999 Columbine shooting. Kevin Oliver/StoryCorps hide caption
A student leaves Columbine High School late Tuesday in Littleton, Colo. The school was closed Wednesday while authorities looked for a woman they said presented a credible threat ahead of the 20th anniversary of the mass shooting there. David Zalubowski/AP hide caption
Columbine High School Principal Frank DeAngelis at a candlelight vigil at the Columbine Memorial at Clement Park near Littleton, Colo., in April of 2009. Chris Schneider/AP hide caption
Oklahoma City Bombing: The Albert P. Murrah Federal Building shows the devastation caused by a fuel and fertilizer truck bomb on April 19, 1995. The blast killed 168 people and injured more than 500. Bob Daemmrich/AFP/Getty Images hide caption
Police gathered at the east entrance of Columbine High School in Littleton, Colo., on April 20, 1999, after two teen students went on a shooting rampage before turning the weapons on themselves. Mark Leffingwell/AFP/Getty Images hide caption
This April 20, 2011, image taken from surveillance video and provided by the FBI shows a "person of interest" sought in a fire and the planting of a pipe bomb and two propane tanks near a food court at the Southwest Plaza Mall in Littleton, Colo. AP hide caption