Children are among the dozens reported dead so far in Moore, the Oklahoma City suburb ravaged by Monday's tornado that packed winds of up to 320 kilometres per hour.
Tornado tears apart Oklahoma community
At least 24 people were killed, including at least seven children, and those numbers were expected to climb.
As the sun rose over the shattered community of Moore, the state medical examiner's office cut the estimated death toll by more than half but warned that the number was likely to climb again.
The spokeswoman Amy Elliott 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," Ms Elliott said. "I truly expect that they'll find more today."
Authorities initially said as many as 51 people were dead, including 20 children. Other estimates put the death toll as high as 91.
The US president, Barack Obama, said he instructed his disaster response team to get tornado victims in the south-west state of Oklahoma everything they need "right away".
Mr Obama called the tornado "one of the most destructive tornadoes in history", saying the extent of the damage was still unknown.
New search-and-rescue teams moved in at dawn yesterday, taking over from the 200 or so emergency responders who worked all night. A helicopter shined a spotlight from above to aid in the search.
The fire chief, Gary Bird, said fresh teams would search the whole community at least two more times to ensure that no survivors - or any of the dead - were overlooked. Crews painted an X on each structure to note it had been checked.
"That is to confirm we have done our due diligence for this city, for our citizens," Mr Bird said.
The community of 56,000 people, 16 kilometres south of Oklahoma City, braced for another long, harrowing day.
"As long as we are here ... we are going to hold out hope that we will find survivors," said Trooper Betsy Randolph, a spokeswoman for the Oklahoma Highway Patrol.
More than 120 people were being treated at hospitals, including about 50 children.
Other search-and-rescue teams focused their efforts at Plaza Towers Elementary, where the storm ripped off the school's roof, knocked down walls and turned the playground into a mass of twisted plastic and metal as students and teachers huddled in hallways and bathrooms.
Mary Fallin, Oklahoma's governor, said she watched up close late on Monday as rescuers tried to find people in the wreckage of the school.
"It was an incredible sight to see how big the debris field was and how much destruction there was," Ms Fallin said. "It would be remarkable for anyone to survive."
Children from the school were among the dead, but several students were pulled alive from under a collapsed wall and other heaps of mangled debris.
Rescue workers passed the survivors down a human chain of parents and neighbourhood volunteers. Parents carried children in their arms to a triage centre in the car park - some students looked dazed, others terrified.
Officials were still trying to account for a handful of children not found at the school who may have gone home early with their parents, Mr Bird said.
The tornado also grazed a theatre, and levelled countless homes. Authorities were still trying to determine the full scope of the damage.
Roofs were torn off houses, exposing metal rods left twisted like pretzels. Cars sat in heaps, crumpled and sprayed with caked-on mud. Insulation and siding was smashed up against the sides of any walls that remained standing. Yards were littered with pieces of wood, nails and pieces of electric poles.
The National Weather Service issued an initial finding that the tornado was an EF-4 on the enhanced Fujita scale, the second-most powerful type of twister. It estimated that the twister was almost a kilometre wide.
The state's Storm Prediction Centre forecast more stormy weather yesterday, predicting golf ball-sized hail, powerful winds and isolated, strong tornadoes in parts of Texas, Arkansas, Louisiana and Oklahoma. The area at risk did not include Moore.
In video of the storm, the dark funnel cloud could be seen marching slowly across the green landscape. As it churned through the community, the twister scattered shards of wood, awnings and glass all over the streets.
Monday's tornado loosely followed the path of a killer twister that slammed the region with 480 kph winds in May 1999. It was the fourth tornado to hit Moore since 1998.
The 1999 storm damaged 600 homes and about 100 businesses. Two or three schools were also hit, but "the kids were out of school, so there were no concerns", said the city official Steve Eddy.
At the time of Monday's storm, the city council was meeting. Local leaders watched the twister approaching on television before taking shelter in the bathroom.
"We blew our sirens probably five or six times," Mr Eddy said. "We knew it was going to be significant, and there were a lot of curse words flying."
Betty Snider, 81, scrambled to go inside with her son and husband. She put her husband, who recently had a stroke, in a bathroom, but there wasn't room for both of them. So she and her son huddled in a hallway.
"That is the loudest roar I've ever heard in my life," she said.
She said she didn't have time to do anything. She couldn't duck, couldn't cover her ears, couldn't find another place to hide.
Ms Snider was in Moore for the 1999 tornado but said this was the closest a twister had ever come to her house, which was still standing.
"We thought that was bad. But this is unbelievable," said her daughter, Phyllis Boyer, 61.
Monday's twister also came almost exactly two years after an enormous twister ripped through the city of Joplin, Missouri, killing 158 people and injuring hundreds more.
That May 22, 2011, tornado was the deadliest in the United States since modern tornado record keeping began in 1950, according to the National Oceanic and Atmospheric Administration. Before Joplin, the deadliest modern tornado was June 1953 in Flint, Michigan, when 116 people died.