A tornado of rare power, that may be among the largest and most destructive in US history, has roared through an Oklahoma City suburb, killing at least 51 people, flattening neighbourhoods with winds up to 200mph and destroying at least one school.
Officials said children from the school were among the dead and the overall death toll was expected to rise, an Oklahoma Medical Examiner's Office spokeswoman said.
Rescuers combed through the debris as darkness fell and rain began to fall. More than 140 people were being treated at hospitals, including about 70 children, some in a critical condition.
"Hearts are broken" for parents looking for their children, governor Mary Fallin said.
The ferocious storm - less than 1% of all tornadoes reach such wind speed - ripped through the suburb of Moore in a Midwest region of the US known as Tornado Alley. Street after street lay in ruins and cars and trucks were left crumpled.
The National Weather Service estimated that the tornado reached up to half a mile wide and was an EF-4 on the enhanced five-point Fujita scale, the second most powerful type of twister.
In video footage, the dark funnel cloud moved slowly across the landscape for more than half an hour, scattering shards of wood, pieces of insulation, shingles and glass over the streets.
The focus quickly turned to Plaza Towers Elementary School, where the storm tore off the roof, knocked down walls and turned the playground into a mass of twisted plastic and metal.
"You could see the debris, like pieces of shingles and insulation and stuff like that, rotating around it," said Chris Calvert, who saw the tornado from about a mile away.
Several children were pulled alive from the rubble. Rescue workers passed them down a human chain to a triage centre in the car park.
As night fell, emergency workers crawled through the ruins, looking for people. Crews used jackhammers and sledgehammers to tear away concrete. Bulldozers were getting stuck in the mud.
James Rushing, who lives opposite, heard reports of the approaching tornado and believing he would be safer there, ran to the school, where his five-year-old foster son Aiden is a pupil.
"About two minutes after I got there, the school started coming apart," he said. The pupils were put in the restroom.
After the tornado passed, Tiffany Thronesberry said she got a panicked call from her mother, Barbara Jarrell.
"I got a phone call from her screaming, 'Help, help! I can't breathe. My house is on top of me!"' Ms Thronesberry said. She hurried to her mother's house, where first responders had already pulled her out with cuts and bruises.
Dangers remained. Downed power lines and open gas lines posed a risk, police captain Dexter Nelson said. Television footage showed at least one fire in the debris.
The same suburb was hit hard by a tornado in 1999. That storm produced the highest winds ever recorded near the Earth's surface - 302mph.
"Oklahoma City has had more tornado strikes than any other city in the United States," the city government's website says.
President Barack Obama called Ms Fallin to express his concern.
A man with a megaphone stood near a Catholic church and called out the names of surviving children as parents waited nearby, hoping to hear their sons' and daughters' names.
Don Denton had not heard from his two sons since the tornado hit the town, but the man who had endured six back surgeries and has a severe limp said he walked about two miles as he searched for them.
As reports of the storm came in, Mr Denton's 16-year-old son texted him, telling him to call.
"I was trying to call him, and I couldn't get through," he said. Eventually,his sons spotted him in the crowd, fine, but upset to hear that their grandparents' home was destroyed.
President Barack Obama declared a major disaster and ordered government aid to supplement state and local recovery efforts.
The Federal Emergency Management Agency has sent a special team to Oklahoma's emergency operations centre to help out and dispatch resources.
Joplin sends expert help
The city of Joplin, Missouri, devastated by a tornado two years ago, is sending a team of safety experts to Moore.
The Joplin tornado killed 158 people and injured hundreds more.
Yesterday Joplin put together a team of about a dozen police and firefighters to assist rescue efforts in Moore.
Joplin city manager Mark Rohr said his community remembered the help it received in 2011 and felt an obligation to lend a hand in Moore.
The team from Joplin will help determine areas in which Moore needs further help and will also work to provide other assistance, Mr Rohr said.
Twister tracked 1999 tornado path
The monster tornado in suburban Oklahoma City loosely followed the path of a killer twister that slammed the region in May 1999.
The National Weather Service estimated that the storm that struck Moore yesterday had wind speeds of up to 200mph and was at least half a mile wide. The 1999 storm had winds clocked at 300mph, according to the weather service website, and it destroyed or damaged more than 8,000 homes, killing at least two people.
Kelsey Angle, a weather service meteorologist in Kansas City, Missouri, said was unusual for two such powerful tornadoes to track roughly the same path. The 1999 twister was part of a two-day outbreak sweeping mostly across central Oklahoma - similar to the past two days.
The weather service has tentatively classified the Moore twister's wind speeds as an EF4 on a five-point scale. Less than 1% of all tornadoes reach EF4 or EF5.
The thunderstorm developed in an area where warm moist air rose into cooler air. Winds in the area caused the storm to rotate, and that rotation promoted the development of a tornado. The most destructive and deadly tornadoes develop from rotating thunderstorms.
The biggest known tornado was nearly two and a half miles wide at its peak width, which the weather service describes as near the maximum size for a tornado. It struck Hallam, Nebraska, in May 2004.
The deadliest tornado, which struck on March 18 1925, killed 695 people in Illinois, Missouri and Indiana.
Deaths from twisters have been declining in recent years because of improved forecasts and increased awareness by people living in tornado-prone areas, especially in smaller and rural communities.
Are you in the area? Have you been affected by the tornadoes?
10 deadliest tornadoes in US since 1900
- 695 deaths. March 18, 1925, in Missouri, Illinois and Indiana.
- 216 deaths. April 5, 1936, in Tupelo, Mississippi.
- 203 deaths. April 6, 1936, in Gainesville, Georgia.
- 181 deaths. April 9, 1947, in Woodward, Oklahoma.
- 158 deaths. May 22, 2011, in Joplin, Missouri.
- 143 deaths. April 24, 1908, in Amite, Louisiana, and Purvis, Mississippi.
- 116 deaths. June 8, 1953, in Flint, Michigan.
- 114 deaths. May 11, 1953 in Waco, Texas.
- 114 deaths. May 18, 1902 in Goliad, Texas.
- 103 deaths. March 23, 1913, in Omaha, Nebraska.
Source: National Oceanic and Atmospheric Administration.
How to help
In the United States the Oklahoma Red Cross is asking people to donate by texting Red Cross to 90999 – that counts for a $10 donation. The organisation has a number of shelters open for those displaced by the storm.
You can also donate to the Salvation Army, if in the United States, by phone: 800-725-2769 -- or online at: donate.salvationarmyusa.org