An untouched chalkboard from 1917 was found behind a classroom wall at Emerson High School in Oklahoma City. Jacob McCleland/KGOU hide caption

itoggle caption Jacob McCleland/KGOU

A general view of downtown Oklahoma City as basketball fans gather outside Oklahoma City Arena. The once run-down area has undergone a major transformation over the past 20 years. Ronald Martinez/Getty Images hide caption

itoggle caption Ronald Martinez/Getty Images

Spectators bow their heads during a moment of silence during a ceremony to commemorate the 20th anniversary of the Oklahoma City bombing at the Oklahoma City National Memorial, on Sunday. Sue Ogrocki/AP hide caption

itoggle caption Sue Ogrocki/AP

Phuong Nguyen, 55, and her son, Chris, who survived the bombing of the Alfred P. Murrah Federal Building in Oklahoma City on April 19, 1995. StoryCorps hide caption

itoggle caption StoryCorps

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