The storm system that generated an F-4 tornado in Moore, Oklahoma is pictured in this May 20, 2013 NASA photo.
Tornado, Oklahoma City, Oklahoma
Massive Tornado Hits Oklahoma
1/10A man stands among the wreckage in Moore, Oklahoma, May 20, 2013
2/10Storm chaser Brad Mack records the tornado in Oklahoma.
3/10The storm system that generated an F-4 tornado in Moore, Oklahoma is pictured in this May 20, 2013 NASA photo.
4/10John Warner surveys the damage near a friend's mobile home in the Steelman Estates Mobile Home Park, destroyed in a tornado, near Shawnee, Oklahoma, May 20, 2013.
5/10A fire burns in the Tower Plaza Addition following a tornado in Moore, Oklahoma, May 20, 2013.
6/10Moore police dig through the rubble of the Plaza Towers Elementary School following a tornado in Moore, Oklahoma, May 20, 2013.
7/10This photo provided by KFOR-TV shows homes flattened outside Moore, Oklahoma, May 20, 2013.
8/10A tornado moves past homes in Moore, Oklahoma, May 20, 2013.
9/10A woman carries her child through a field near the collapsed Plaza Towers Elementary School in Moore, Oklahoma, May 20, 2013.
10/10Seven-year-old Katrina Ash holds a doll as she waits in the back of a truck with her grandfather, Michael Bowen, after a tornado ripped through their neighborhood near Dale, Oklahoma, May 19, 2013.