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

itoggle caption Bob Daemmrich/AFP/Getty Images

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

itoggle caption Mark Leffingwell/AFP/Getty Images

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

itoggle caption AP