A man and two children exit a shelter after debris was cleared from on top, after a huge tornado struck Moore, Oklahoma, near Oklahoma City, May 20, 2013.

People look through the wreckage of their neighborhood after a tornado struck Moore, Oklahoma, May 20, 2013. A huge tornado with winds of up to 200 miles per hour (320 kph) devastated the Oklahoma City suburb of Moore on Monday, ripping up at least two elementary schools and a hospital and leaving a wake of tangled wreckage.

A rescue worker checks the rubble in a residential area in Moore, Oklahoma May 21, 2013 after a massive tornado struck the area May 20.

An aerial view of an entire neighborhood destroyed by Monday's tornado is shown Tuesday, May 21, 2013, in Moore, Okla.

A damaged police car is seen after a tornado struck Moore, Oklahoma, May 20, 2013. A 2-mile-wide tornado tore through the Oklahoma City suburb of Moore on Monday, killing at least 51 people while destroying entire tracts of homes, piling cars atop one another, and trapping two dozen school children beneath rubble.

Rescue workers help free one of 15 people trapped in a medical building at the Moore hospital complex after a tornado tore through the area of Moore, Oklahoma May 20, 2013.