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

Mine helmets and painted crosses sat at the entrance to Massey Energy's Upper Big Branch coal mine on April 5, as a memorial to the 29 miners killed there one year earlier. Jeff Gentner/AP hide caption

itoggle caption Jeff Gentner/AP