As the school day was ending, parents in the Oklahoma City suburb of Moore were forced to make an impossible calculation – rush to schools to gather up their children and risk being caught in the open, or trust that the reinforced walls of the school buildings would shelter them.

A tornado was coming, one that may be the most destructive to ever strike the United States. There would be little respite.

The death toll from the savage tornado that hit Moore on the southern fringe of Oklahoma City has been revised down to 24 people, including nine children.

A spokeswoman for the Oklahoma Medical Examiners Office, Amy Elliot, made the announcement just after 11am local time, as recovery and rescue efforts continued across the path of the disaster.

Ms Elliot said last night’s death toll of 51 may have included some who were double counted during the confusion of the early stages of the recover, while difficulties with communications could have made things worse.

The New York Times had reported that the death toll was going to reach 90.

The tornado injured at least 237 people, demolished three schools and thousands of homes as it carved a swathe over 35 kilometres long over a terrifying 40 minutes on Monday afternoon local time.

INTERACTIVE: See the frequency and intensity of tornadoes in the United States over the past 12 years with this interactive map.

From the air over Oklahoma earlier today the path was clearly visible – a long brown dirty stain across the soaking green fields that ploughed unswervingly from farmland through the industrial fringe and into the suburbs.

Through the morning, as recovery efforts continued, sheets of rain fell, punctuated by lightening and booming thunder.

Seven of the nine children are understood to have died in the Plaza Towers Elementary School, which was struck by the tornado as the children prepared to go home at the end of the day on Monday. Many have been identified and their remains were being returned to their families, Ms Elliot said.

As the storm hit Plaza Towers many children from the third grade were caught in a corridor with nothing to hold onto but the walls and floor. It was reported that one or more teachers lay across children to shield them, an act praised by the President, Barack Obama when he addressed the nation about the storm earlier today.

“Our gratitude is with teachers who gave their all to shield their children; with the neighbours, first responders, emergency personnel who raced to help as soon as the tornado passed, and with all of those who, as darkness fell, searched for survivors through the night,” he said.

VIDEO: Listen to Fairfax US correspondent Nick O'Malley as he walks through the tornado devastated suburb of Moore, Oklahoma City.

That search continues along the path of the storm today, where houses in some areas have simply ceased to exist, and in others are simply piles of detritus. Though it is not threatened today by further tornados, Moore and nearby Newcastle, also hit, face further storms and rains.

The tornado that hit on Monday afternoon followed the path of two terrible tornadoes, those that struck in 2003 and 1999. During the 1999 storm the strongest wind recordings made on Earth were taken - 486 kilometres an hour.

VIDEO GALLERY: YouTube footage of the tornado terrror

Veterans of the 1999 disaster say the tornado that hit on Monday was worse. Parts of Moore, in the southern suburbs of Oklahoma City, were obliterated, with everything, even grass, sucked away by the wind.

The tornado was categorised EF5, the most powerful on a scale from one to five. 

The funnel was in contact with the ground for a terrible 40 minutes.

The first warnings came about 2.40pm. A menacing weather system, one that had killed two the previous day, was about to turn violent again.

'THE SCHOOL STARTED COMING APART'

By 2.56pm the funnel had hit the ground outside Oklahoma City at Newcastle. Sirens sounded. In the city, staff of the House of Representatives fled to basements as the media broadcast storm warnings. Residents were told to shelter in their basements.

But the ground is hard around Oklahoma City and many residents don't have storm shelters. Those who could not get underground tried to take shelter in bathtubs from what might have been the most destructive tornado recorded.

At Plaza Towers Elementary, children in the fifth and sixth grades had already been evacuated to a church. It is thought those in kindergarten, first and second grades were sheltering but many from the third grade are thought to have been in the corridor when the tornado slammed into the school.

''I had to hold onto the wall to keep myself safe because I didn't want to fly away in the tornado,'' one girl told a local television station, KFOR. Others described a teacher lying across children to protect them. All survived.

James Rushing, who lives across the street, ran to the school to take shelter, thinking the building would be safer than his home. ''About two minutes after I got there, the school started coming apart,'' he told Associated Press.

A boy rescued from the corridor told a reporter: ''It was scary and a lot of my friends were still there when I left.''

Struggling to describe the scene, KFOR's Lance West said: ''I have never seen anything like this in my 18 years covering tornadoes here in Oklahoma City. This is without question the most horrific,'' before his voice broke.

One survivor pacing the streets told of trying to find loved ones and of not being able even to locate his home among the stripped driveways and slabs. Another man managed to find his grandmother wandering dazed, carrying her dog.

At the school, rescuers were confronted by a three-metre sea of debris as they worked through the dusk and into the night to dig out the children and staff.

Shattered-looking survivors were passed along human chains. A mother and her seven-month-old baby were found dead, having failed to find shelter in a large freezer.

By 9pm it became clear that what had been a rescue was becoming a recovery operation. Rescuers began using more heavy equipment to work faster. News broke that some of the children, perhaps seven, had been found dead in a pool of water.

As the operation continued through the night, forecasters warned that the weather was not expected to improve and that the tornado season had a month to run in what is known as twister ally.

Meanwhile, Americans prepared themselves, yet again, for the sight of a search for bodies in wrecked homes, and of dead children being removed from a primary school.