Search and rescue crews worked through the night in Oklahoma after a tornado with winds of 320 kilometers an hour caused massive destruction late Monday.
At least 51 people were killed, including at least 20 children, and those numbers are expected to climb. A spokeswoman for the state medical examiner's office says as many as 40 more fatalities may be reported. At least 240 people were injured.
The twister demolished an elementary school and reduced homes to piles of splintered wood in Moore, a suburb of Oklahoma City.
Rescue worker pulled several children alive out of the rubble of two schools that were hit.
Oklahoma Governor Mary Fallin said "hearts are broken'' for parents looking for their children. She deployed the state National Guard and extra police to assist with rescue operations.
President Barack Obama declared a major disaster in Oklahoma as a result of the storm. He ordered the Federal Emergency Management Agency to provide any assistance that Oklahoma needs.
The National Weather Service said the magnitude of this tornado was an EF-4, the second most-powerful type of twister on the five-level scale that measures a storm's destructive power.
Moore, Oklahoma also was hit hard by a deadly tornado in 1999.
The tornado Monday is the deadliest in the United States since 2011, when a storm in the central community of Joplin, Missouri killed 161 people.