Story highlights "How do we explain this to the kids?" asks a mother

At least 145 people are injured at area hospitals

Oklahoma Gov. Mary Fallin promises support

Rescue workers are digging through debris at the school

Rescue workers raced against time and darkness Monday night looking for survivors after a powerful tornado blasted an area outside of Oklahoma City, leveling homes and leaving at least 51 people dead.

At least 20 of the dead were children, including at least seven from Plaza Towers Elementary School in Moore, which lay directly in the path of the monster storm's wall of wind.

Seventy-five students and staff members had been huddled at the school when the tornado hit, CNN affiliate KFOR reported.

As nightfall descended, determined searchers in hard hats dug in the debris for those possibly trapped, but authorities described the work as a recovery, not rescue, effort. Search lights illuminated their efforts.

A father of a third-grader still missing sat quietly on a stool. Tears fell from his face as he waited for news.

Photos: Photos: Tornadoes strike Midwest Photos: Photos: Tornadoes strike Midwest Tornadoes wreak havoc in Midwest – Rescuers search through rubble in Shawnee, Oklahoma, on Monday, May 20. A tornado outbreak hit in the Midwest and Plains on Sunday and Monday, the deadliest hitting Moore, Oklahoma, on Monday. Hide Caption 1 of 18 Photos: Photos: Tornadoes strike Midwest Tornadoes wreak havoc in Midwest – Massive piles of debris cover the ground after a powerful tornado ripped through Moore, Oklahoma, on May 20. View photos related to the Moore tornado. Hide Caption 2 of 18 Photos: Photos: Tornadoes strike Midwest Tornadoes wreak havoc in Midwest – A deadly tornado destroys cars and demolishes structures in the town of Moore, near Oklahoma City, on Monday, May 20. Hide Caption 3 of 18 Photos: Photos: Tornadoes strike Midwest Tornadoes wreak havoc in Midwest – A volunteer helps clean up a mobile home on May 20 after it was overturned on a day earlier near Shawnee, Oklahoma. Hide Caption 4 of 18 Photos: Photos: Tornadoes strike Midwest Tornadoes wreak havoc in Midwest – Jean McAdams' mobile home near Shawnee, Oklahoma, lies overturned on May 20. Hide Caption 5 of 18 Photos: Photos: Tornadoes strike Midwest Tornadoes wreak havoc in Midwest – President Barack Obama talks on the phone with Oklahoma Gov. Mary Fallin from the Oval Office on May 20. The president expressed his concern for those who have been affected by the severe weather. Hide Caption 6 of 18 Photos: Photos: Tornadoes strike Midwest Tornadoes wreak havoc in Midwest – Tom and Ronda Clark get help with cleanup on May 20, after their property near Shawnee was damaged by a tornado on May 19. Hide Caption 7 of 18 Photos: Photos: Tornadoes strike Midwest Tornadoes wreak havoc in Midwest – Lonnie Langston says his garage was swept off the concrete pad next to his house by a tornado near Shawnee. Hide Caption 8 of 18 Photos: Photos: Tornadoes strike Midwest Tornadoes wreak havoc in Midwest – Shawnee residents embrace on May 20 as they search through the remains of their home. Hide Caption 9 of 18 Photos: Photos: Tornadoes strike Midwest Tornadoes wreak havoc in Midwest – A home in Shawnee sits in ruin after being hit by a tornado on Sunday, May 19. Hide Caption 10 of 18 Photos: Photos: Tornadoes strike Midwest Tornadoes wreak havoc in Midwest – A twister stretches toward the ground near South Haven, Kansas, on May 19. Hide Caption 11 of 18 Photos: Photos: Tornadoes strike Midwest Tornadoes wreak havoc in Midwest – Residents repair the roof of a neighbor's damaged house after a tree fell on it in Shawnee on May 19. Hide Caption 12 of 18 Photos: Photos: Tornadoes strike Midwest Tornadoes wreak havoc in Midwest – A woman waits to be allowed back to her home after a tornado swept through Shawnee on May 19. Hide Caption 13 of 18 Photos: Photos: Tornadoes strike Midwest Tornadoes wreak havoc in Midwest – Storm chaser and videographer Brad Mack records a tornado touchdown in South Haven, Kansas, on May 19. Hide Caption 14 of 18 Photos: Photos: Tornadoes strike Midwest Tornadoes wreak havoc in Midwest – A tractor-trailer lies on its side on Interstate 40 while another is broken open on the road below after falling from the overpass after a tornado strike near Highway 177 north of Shawnee on May 19. Hide Caption 15 of 18 Photos: Photos: Tornadoes strike Midwest Tornadoes wreak havoc in Midwest – A tornado touches down near Wichita, Kansas, on Sunday, May 19. Hide Caption 16 of 18 Photos: Photos: Tornadoes strike Midwest Tornadoes wreak havoc in Midwest – Debris from a mobile home park west of Shawnee litters the ground on May 19. An estimated 300 homes were damaged or destroyed across Oklahoma, Red Cross spokesman Ken Garcia said. Hide Caption 17 of 18 Photos: Photos: Tornadoes strike Midwest Tornadoes wreak havoc in Midwest – Lightning strikes in Clearwater, Kansas, on May 19. Hide Caption 18 of 18

JUST WATCHED Tornado leaves path of destruction Replay More Videos ... MUST WATCH Tornado leaves path of destruction 01:56

JUST WATCHED Tornado hits Oklahoma school Replay More Videos ... MUST WATCH Tornado hits Oklahoma school 01:02

JUST WATCHED Man found neighbor trapped after tornado Replay More Videos ... MUST WATCH Man found neighbor trapped after tornado 02:05

"I'm speechless. How did this happen? Why did this happen?" asked Norma Bautista, whose son, a student, survived.

"How do we explain this to the kids? How are they going to wake up tomorrow and everything's missing -- the school, these houses, their friends," she said. "In an instant, everything's gone."

After the ear-shattering howl of the killer storm subsided, survivors along the miles of destruction emerged from shelters to see an apocalyptic vision -- the remnants of cars twisted and piled on each other to make what had been a parking lot look like a junk yard.

Block after block of homes were gone. Bright orange flames flew from a structure that was blazing even as rain continued to fall.

"Our worst fears are becoming realized this afternoon," said Bill Bunting, with the National Oceanic and Atmospheric Administration's Storm Prediction Center.

"We certainly hope everyone heeded the warnings, but it's a populated area and we just fear that not everyone may have gotten the word," he said.

Bodies of those killed in the storm were being sent to Oklahoma's office of the chief medical examiner, said the office's Amy Elliott. At least 145 people were reported injured at three area hospitals.

The preliminary rating of damage created by the tornado is at least EF4 (winds 166 to 200 mph) -- the second-most severe classification on a scale of zero to five -- according to the National Weather Service.

The tornado was estimated to be at least two miles wide at one point as it moved through Moore, KFOR reported.

Lando Hite, shirtless and spattered in mud, told the CNN affilaite about the storm hitting the Orr Family Farm in Moore, which had about 80 horses.

"It was just like the movie 'Twister,'" he said, standing amid the debris. "There were horses and stuff flying around everywhere."

'This is not over yet'

Speaking to reporters Monday night, Oklahoma Gov. Mary Fallin said that officials are doing everything they can to help find people who may be lost or injured.

Sixty-five patients, including 45 children and 20 adults, were at Oklahoma University (OU) Medical Center, said spokesman Scott Coppenbarger.

Injuries ranged from minor to critical.

JUST WATCHED See cell phone video of tornado Replay More Videos ... MUST WATCH See cell phone video of tornado 00:45

JUST WATCHED Myers: Like a bowling ball went through Replay More Videos ... MUST WATCH Myers: Like a bowling ball went through 01:12

JUST WATCHED Vine videos show tornado devastation Replay More Videos ... MUST WATCH Vine videos show tornado devastation 02:53

JUST WATCHED Mapping tornado's path of destruction Replay More Videos ... MUST WATCH Mapping tornado's path of destruction 02:48

Moore Medical Center in Oklahoma was evacuated after it sustained damage, a hospital spokeswoman said.

All patients were being sent to Norman Regional Hospital and Healthplex Hospital, and residents injured in the storm were being told to go to those centers as well.

Between those two facilities, 80 patients were being treated for signs of trauma, lacerations and broken bones, among other injures.

The tornado also disrupted roads, piling them high with debris and complicating both travel and communication.

Interstate 35 in Moore was closed, Oklahoma Department of Transportation spokesman Cole Hackett said. Crews were heading to the north-south highway to start the cleanup process.

"People are trapped. You are going to see the devastation for days to come," said Betsy Randolph, spokeswoman for Oklahoma Highway Patrol. She did not say how many people were stuck.

More than 38,000 electricity customers in Oklahoma are without power, according to local power providers.

Even as authorities worked to wrap their heads around the damage, NOAA's Bunting warned the worst may be yet to come.

"These storms are going to continue producing additional tornadoes. They'll also produce some very, very large hail, perhaps larger than the size of baseballs. We're also concerned that there may be an enhanced and widespread damaging wind threat with storms as they merge together," he said.

"As bad as today is, this is not over yet."

Oklahoma resident: 'It's just all gone'

The severe weather came after tornadoes and powerful storms ripped through Oklahoma and the Midwest earlier Monday and on Sunday.

Forecasters had said that the destructive weather, which killed at least two people, was perhaps just a preview.

Before Monday afternoon's devastation, residents in areas hard hit by weekend storms were combing through rubble where their homes once stood.

"My mind is, like, blown, completely blown," said Jessie Addington, 21, who found that few pieces of her childhood home in Shawnee, Oklahoma, were still standing Monday.

Addington, who now lives in a nearby town, said her mother huddled in the mobile home's bathroom when the weekend storm hit. But the tornado still tossed her around like a rag doll, leaving her bruised.

When Addington arrived, she was shocked to find the neighborhood where she had lived for 17 years reduced to ruins.

"I'm feeling cheated, to be honest," she said, "like, it's just all gone."

An estimated 300 homes were damaged or destroyed across Oklahoma in weekend weather, Red Cross spokesman Ken Garcia said.

Two men, both in their 70s, were confirmed dead as a result of an earlier tornado that hit Shawnee, said Elliott, the spokeswoman for the state medical examiner's office.

As many as 28 tornadoes were reported in Oklahoma, Kansas, Illinois and Iowa, according to the National Weather Service, with Oklahoma and Kansas the hardest hit. Some of those reports might have been of the same tornado.

A combination of factors -- including strong winds and warm, moist air banging against dry air -- means severe weather could continue sweeping across a wide swath of the United States for days, Petersons said.

"Keep in mind we have all the ingredients out there that we need," she said.

Tornado watches were in effect for portions of southeastern Kansas, western and central Missouri, northwest Arkansas, central and eastern Oklahoma and northwestern Texas until 10 p.m. (11 p.m. ET).