Doyle Rice, William M. Welch, Michael Winter and Doug Stanglin USA Today
A massive, mile-wide tornado with winds up to 200 mph spent 40 minutes on the ground as it devastated homes, schools and businesses across southern Oklahoma City and its suburbs Monday afternoon.
The state medical examiner confirmed at least 51 deaths and said the toll was expected to rise.
WATCH, SHARE: Live coverage of Oklahoma City tornado
Catastrophic damage was reported in Moore, where two elementary schools were destroyed, including one that took a direct hit. Several children were pulled alive from the rubble of Plaza Towers Elementary, but there were no immediate reports of rescues or casualties at Briarwood Elementary.
Three hospitals reported treating at least 120 injured, including some children pulled from the Plaza Towers school.
More than 60 patients were treated for tornado injuries at Norman Regional Medical Center.
FULL COVERAGE : Twisters devastate Okla. City suburbs
The twister heavily damaged one hospital in Moore, ripping off its roof, and a water treatment was knocked offline. Residents and businesses in southeastern Oklahoma City were advised to stop using water.
The preliminary rating of the tornado that hit Moore at 2:53 p.m. CT is at least an EF-4, which means wind speeds of from 166 to 200 mph, the National Weather Service said.
On May 3, 1999, a record-setting EF-5 tornado obliterated the city of 55,000 with winds measured at 318 mph, the highest ever on the earth's surface. The storm killed 36 people, injured hundreds, and caused about $1 billion in damages.
The National Weather Service in Norman, Okla., said a tornado warning was in effect Monday afternoon for 16 minutes before the twister developed.
VIDEO: Timelapse of tornado
Rescuers are "going house to house and block to block to try and find any survivors that are out there and trapped,'' said state emergency management spokesman Jerry Lojka.
"We can only imagine that there are still many others there that are unaccounted for,'' he said.
Lojka said emergency management officials were working from an underground command center in Oklahoma City and did not yet know how many students were in the two elementary schools in Moore that were destroyed.
Earlier in the day, Oklahoma, Texas, Kansas, Arkansas and Missouri had been under a tornado watch, meaning that conditions were favorable for tornadoes to develop within the next few hours.
The National Weather Service said it was tracking "a large and extremely dangerous tornado'' just west of Moore, The storm was moving to the northeast, and forecasters said they expected "large, destructive hail up to tennis ball size.''
Video aired by KFOR-TV showed a massive, dark funnel-shaped cloud over the area and, later, scenes of massive destruction. Entire neighborhoods were flattened.
On Sunday, a tornado packing winds as high as 200 mph, left two people dead in Oklahoma. Tornadoes and high winds injured more than 20 in the region.
The body of 79-year-old Glen Irish was found in an open area of the neighborhood after a twister slammed into Shawnee, Okla., leveling a mobile home park.
The Oklahoma medical examiner on Monday confirmed the second fatality, Billy Hutchinson, 76.
Both victims were from Shawnee, but it was not immediately clear if both lived in the Steelman Mobile Home Park, which was destroyed.
"You can see where there's absolutely nothing, then there are places where you have mobile home frames on top of each other, debris piled up," Pottawatomie County Sheriff Mike Booth said after surviving damage. "It looks like there's been heavy equipment in there on a demolition tour.
"It's pretty bad. It's pretty much wiped out," Booth said.
More than 60 million Americans are at risk of severe storms Monday, with the primary targets including Oklahoma, Kansas, Missouri and Arkansas, the National Weather Service's Storm Prediction Center warned.
"Damaging wind gusts, large hail and tornadoes are possible in all areas," Weather Channel meteorologist Kevin Roth said.
Besides Oklahoma City, other cities at risk from severe weather Monday included Tulsa, St. Louis, Cedar Rapids, Iowa, and Minneapolis, said AccuWeather meteorologist Meghan Evans. Chicago, Detroit, Dallas and Indianapolis also are in the danger zone.
Sunday, there were 24 reports of tornadoes in five states, the Storm Prediction Center said. "In what has otherwise been a quiet spring for tornadoes, May 19 appeared to have been the second-most active day for tornadoes in the nation so far in 2013," Weather Channel meteorologist Jon Erdman said.
So far this year -- not including this most recent five-day outbreak -- severe storms have caused $3.5 billion in economic losses in the USA, says meteorologist Steve Bowen of global reinsurance firm Aon Benfield. Bowen says. Of that $3.5 billion, at least $2 billion was covered by insurance.
"By the time the current storm system finally winds down by the middle of this week, I wouldn't be surprised if this ends up as the costliest U.S. natural disaster event we've seen so far in 2013," said meteorologist Steve Bowen of global reinsurance firm Aon Benfield.
"Recent full-year severe weather-related insured losses were roughly $27 billion in 2011 and $15 billion in 2012 - the two costliest years on record," Bowen said. By this definition, "severe" weather means damage from thunderstorms or tornadoes, and does not include damage from hurricanes.
The storms in Oklahoma on Sunday that ripped off roofs and tossed big trucks like toys were part of a severe weather outbreak that stretched from Texas to Minnesota. Twisters were also reported Sunday in Iowa and Kansas.
The killer tornado that flattened portions of Shawnee, Okla., had wind speeds that were estimated as high as 200 mph, according to the National Weather Service.
Across Oklahoma, 21 people were injured, not including those who suffered bumps and bruises and chose not to visit a hospital, said Keli Cain, a spokeswoman for the Oklahoma Department of Emergency Management. Booth said six at Steelman Estates were hurt.
Gov. Fallin declared an emergency for 16 Oklahoma counties.
Interstate 40 was closed by the Oklahoma Highway Patrol after winds overturned semi-tractor trailer trucks and other vehicles, Newsok.com reported.
KFOR-TV showed footage of homes damaged and cars and trucks flipped from highways near Shawnee. Other video showed flashes from electrical transformers blowing out as they were hit by high winds or debris from the tornado near Edmond.
A tornado touched down in Golden City, Mo., early Monday morning and tore through two counties, Barton County Emergency Management Director Tom Ryan told CNN. The number of injuries and extent of damage were not immediately clear.
Sedgwick County, Kan., emergency management director Randy Duncan said officials are grateful for few reports of damage from a tornado that touched down near Wichita Mid-Continent Airport. He told CNN the area emerged "relatively unscathed.''
Forecasters had been warning for days that severe storms were likely across the region.
"I knew it was coming," said Randy Grau, who huddled with his wife and two young boys in their Edmond's home when a tornado hit. He said he peered out his window as the weather worsened and believed he saw a flock of birds heading down the street. "Then I realized it was swirling debris.''
In Iowa, a tornado touched down Sunday about 30 miles west of Des Moines near the town of Earlham, the Des Moines Register reported.
Meteorologist Kurt Kotenberg said a large low-pressure system is parking itself over the middle of the country and "really isn't going to move much over the course of the next few days. ... It's basically going to keep pulling up that nice Gulf (of Mexico) moisture that keeps fueling everything."
The threat of twisters comes less than a week after tornadoes left six dead, dozens injured and hundreds of homes destroyed in Texas and just shy of the two-year anniversary of the Joplin, Mo., twister.
Contributing: The Associated Press