MOORE, Okla. » Oklahoma Gov. Mary Fallin spoke directly to citizens in her devastated state Monday night, saying "we are looking under every single piece of debris" for the missing after a massive tornado tore across parts of Oklahoma City and its suburbs.

"Our hearts are broken for the parents that are wondering about the state of their children that had been in the schools that have been hit today," Fallin said. "I know that there are families wondering where their loved ones are. … We will bring in every single resource that we can."

A giant tornado, a mile wide or more, killed at least 51 people, 20 of them children, flattening homes, flinging cars through the air and crushing at least two schools. Amy Elliott, spokeswoman for the Oklahoma Medical Examiner’s Office, told the Associated Press early today that officials could see as many as 40 more fatalities from the tornado.

The injured flooded into hospitals, and the authorities said many people remained trapped, even as rescue workers struggled to make their way through debris-clogged streets to the devastated suburb of Moore, where much of the damage occurred.

Hospitals reported at least 120 people were being treated, 50 of them children.

Plaza Towers Elementary School in Moore was reduced to a pile of twisted metal and toppled walls. Rescue workers were able to pull several children from the rubble, but on Monday evening crews were still struggling to cut through fallen beams and clear debris amid reports that dozens of students were trapped. At Briarwood Elementary School in Oklahoma City, which borders Moore, cars were thrown through the facade and the roof was torn off.

"Numerous neighborhoods were completely leveled," Sgt. Gary Knight of the Oklahoma City Police Department said by telephone. "Neighborhoods just wiped clean."

Debris and damage to roadways, along with heavy traffic, hindered emergency responders as they raced to the affected areas, Knight said.

A spokeswoman for the mayor’s office in Moore said emergency workers were working to assess the damage. "Please send us your prayers," she said.

Fallin said she had called out the National Guard and declared federal emergencies in 21 counties. She said President Barack Obama had offered assistance and that state officials were already in touch with the Federal Emergency Management Agency.

The tornado touched down at 2:56 p.m., 16 minutes after the first warning went out, and traveled for 20 miles, said Keli Pirtle, a spokeswoman for the National Weather Service in Norman, Okla. It was on the ground for 40 minutes, she said. It struck the town of Newcastle and traveled about 10 miles to Moore, a populous suburb of Oklahoma City.

Pirtle said preliminary data suggested that it was a Category 4 tornado on the Enhanced Fujita scale, which measures tornado strength on a scale of 0 to 5. A definitive assessment will not be available until today, she said.

Moore was the scene of another huge tornado, in May 1999, in which winds reached record speeds that surpassed 300 mph.

Television on Monday showed destruction spread over a vast area, with blocks upon blocks of homes and businesses destroyed. Residents, some partly clothed and apparently caught by surprise, were shown picking through rubble. Several structures were on fire, and cars had been tossed around, flipped over and stacked on top of each other.

Kelcy Trowbridge, her husband and their three young children piled into their neighbor’s cellar just outside of Moore and huddled together for about five minutes, wrapped under a blanket as the tornado screamed above them, debris smashing against the cellar door.

They emerged to find their home flattened, and the family car resting upside down a few houses away. Trowbridge’s husband rushed toward what was left of their home and began sifting through the debris, then stopped, and told her to call the police.

He had found the body of a little girl, about 2 or 3 years old, she said.

"He knew she was already gone," Trowbridge said. "When the police got there, he just bawled."

She said: "My neighborhood is gone. It’s flattened. Demolished. The street is gone. The next block over, it’s in pieces."

Sarah Johnson was forced to rush from her home in Moore to a hospital as the storm raged when her 4-year-old daughter, Shellbie, suffered an asthma attack. With hail raining down, she put a hard hat on her daughter as she raced into the emergency room and hunkered down.

"We knew it was coming — all the nurses were down on the ground so we got down on the ground," Johnson said from the Journey Church in nearby Norman, where she had sought shelter.

At the hospital, she said, she shoved her daughter next to the wall and threw a mattress on top of her. After the storm passed, she said debris and medical equipment were scattered around. She said she and her daughter were safe, but she had yet to find her husband.

Russell Schneider, the director of the National Oceanic and Atmospheric Administration’s Storm Prediction Center in Norman, said the risk of tornadoes throughout the region remained high going into today.