Spokeswoman Keli Pirtle said Tuesday the agency upgraded the tornado from an EF-4 on the enhanced Fujita scale to an EF-5 based on what a damage assessment team saw on the ground.
The weather service uses the word "incredible" to describe the power of EF-5 storms.
The weather service says the tornado's path was 17 miles long and 1.3 miles wide.
Pirtle says Monday's twister is the first EF-5 tornado of 2013.
The death toll from the tornado remains at 24.
Authorities initially said as many as 51 people were dead, including 20 children.
Amy Elliott, a spokeswoman for the medical examiner, said she believes some victims were counted twice in the early chaos of the storm that struck Monday afternoon. Downed communication lines and problems sharing information with officers exacerbated the problem, she said.
"It was a very eventful night," Elliott said. "I truly expect that they'll find more today."
New search-and-rescue teams moved at dawn Tuesday, taking over from the 200 or so emergency responders who had worked all night. A helicopter shined a spotlight from above to aid in the search.
Many houses have "just been taken away. They're just sticks and bricks," Gov. Mary Fallin said, describing the 17-mile path of destruction.
Emergency crews were having trouble navigating neighborhoods because the devastation is so complete, and there are no street signs left standing, Fallin added.
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