UPDATE (May 21, 2013 - 10:00 A.M.)
MOORE, Oklahoma (ABC News) --First responders are in a race against time in the search for any survivors of a devastating tornado that ripped through Moore, Okla., while the medical examiner's office has revised the death toll from 51 to 24, including seven children.
Oklahoma medical examiner spokeswoman Amy Elliot said this morning that she believes some victims were counted twice in the early chaos of the storm.
Two elementary schools were in the path of Monday's tornado, which the National Weather Service gave a preliminary rating of at least EF-4, meaning churning wind speeds of up to 200 mph.
Oklahoma City police spokesman Sgt. Gary Knight said some of the victims were from Plaza Towers Elementary School.
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Moore Mayor Glenn Lewis and National Guard members told ABC News the search-and-rescue operation at the school is now a body-recovery effort.
"The walls were just pancaked, absolutely flattened and the students were just grouped together," Lt. Gov. Todd Lamb told ABC News.
Oklahoma Gov. Mary Fallin tweeted late Monday night that she visited with search crews at the elementary school. "Appreciate their hard work and tireless dedication," she tweeted.
Fallin has also deployed 80 National Guard members to help with search-and-rescue efforts throughout the city.
Authorities said Briarwood Elementary School in Moore received a "direct hit" from the storm and was also destroyed, with its roof and walls blown off.
"A lot of parents started walking, running to Briarwood, and when we got up to Briarwood, it had been just completely destroyed," Moore resident Robert Raymond said. "I'm just happy that I was able to find my son and my family is OK. The scene over there at the school is just catastrophic. I've never seen anything like it before."
Moore resident Andrew Wheeler credits a Briarwood teacher with keeping his son safe as the tornado wrecked havoc on the building as students were preparing for their final days in school before summer vacation.
"The teacher held their heads, and bricks and everything were falling all over the kids. She got her arm injured. One of the other boys on her other side got a big gash in his head, but he's OK," Wheeler said.
A total of 242 patients, including 58 children, were treated at hospitals. Many patients have been treated and discharged while others have been transferred among hospitals.
Kelly Wells, spokeswoman for Norman Regional Health System, which oversees three hospitals in Oklahoma, said lacerations, broken bones, head and neck injuries were the most common.
Moore Medical Center, the only hospital in Moore, sustained major damage and was evacuating all its patients to other hospitals.
Betsy Randolph of the State Highway Patrol asked people not involved in search-and-rescue operations to stay off the roads so first responders can do their job.
"We do still have rescue, search-and-rescue crews throughout this city. Some of the heavily hit areas, they are still searching for people. We still have people that are trapped," she said.
President Obama signed a disaster declaration in Oklahoma and ordered federal aid to supplement state and local recovery efforts in the area affected by severe storms and tornadoes.
The first tornado warning went out around 2:40 p.m. local time and just 16 minutes later a tornado tore a 12-mile gash in Oklahoma from Newcastle to Oklahoma City. Frantic groups of rescuers could be seen digging through debris within minutes after the tornado blew by.
Moore, a community of 41,000 people about 10 miles south of Oklahoma City, saw homes wiped off their foundations and cars tossed like toys on top of nearby buildings. Block after block lay in ruins, reduced to smoking piles of wood and brick.
"I was pulling walls off of people," Moore resident Tomas Earson said. "There were people crawling out from everywhere and anywhere. It's basically just a war zone."
The weather service estimated that the tornado was at least a half-mile wide and says it could have been on the ground for as long as 40 minutes.
Moore resident Melissa Newton said the hail from the tornado was "about the size of golf balls."
As Moore continues to sift through rubble for survivors, millions across the Midwest are once again under the threat of tornadoes. People in northeast Texas all the way to southwest Arkansas have a 10 percent chance of seeing a twister later today.
More than 50 tornadoes ravaged the Midwest this weekend, killing a 79-year-old man in Shawnee, Okla.
Monday's devastation in Oklahoma came almost exactly two years after an enormous twister ripped through the city of Joplin, Mo., killing 158 people and injuring hundreds more.
Joplin City Manager Mark Rohr said his community remembers the assistance it received in 2011 and believes it has an obligation to lend a hand in Moore.
Moore was the site of one of the most destructive tornadoes in U.S. history. An EF-5 tornado ripped through the Oklahoma City-area May 3, 1999, killing 42 people.