Aid groups are mobilizing relief efforts to help victims of the storm. Here, Candice Lopez, left, and Stephanie Davis help clean debris from Thelma Cox's mobile home near Shawnee, Okla., after it was destroyed Monday. Brett Deering/Getty Images hide caption

itoggle caption Brett Deering/Getty Images

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