A massive tornado up to a mile wide chewed through Moore, Okla., a suburb of Oklahoma City, on Monday afternoon, grinding up entire neighborhoods and obliterating an elementary school where students who had huddled in a hallway with their teachers were buried in rubble.

The state medical examiner’s office said late Monday that 51 people, including 20 children, had been confirmed killed in the tornado. “We’re sitting at 51, and the phone calls just keep coming,” said Amy Elliot, a spokeswoman for the office. Early Tuesday, she told the Associated Press that the medical examiner’s office had been told to expect another 40 bodies.

Some children were pulled — wet and dirty but alive — from the shredded Plaza Towers Elementary School. But as darkness fell Monday, dozens of rescuers in hard hats continued to pick through the wreckage looking for children and staffers who might be trapped.

Families were told to convene at two churches to await reunification with their loved ones.

“Our prayers are with you, and we’re working as quickly as we can to work through the debris,” said Oklahoma Gov. Mary Fallin.

Helicopter footage showed a wide path of near-total destruction in a community that had endured a similarly powerful twister 14 years ago. In addition to Plaza Towers, another elementary school, Briarwood, was demolished. All students at that school were accounted for, according to local news reports.

On the Enhanced Fujita damage scale of tornadoes, this was probably a 4 or 5, at the highest ends of violence, with winds reaching 200 miles per hour, said Russell Schneider, director of the National Oceanic and Atmospheric Administration’s Storm Prediction Center in Norman, Okla., about eight miles from the path of the storm.

“It’s a very wide swath of very intense damage, a large number of structures almost totally destroyed,” Schneider said. “That in and of itself is usually indicative of a violent tornado, EF4 or EF5.”

Plaza Towers Elementary School, where scores of students took shelter as the twister approached, was destroyed. Nearby, cars and trucks were heaped on top of one another and homes were reduced to foundations covered with splintered wood.

“I’m sick to my stomach,” said Jayme Shelton, a spokesman for the city of Moore, reached by telephone. “Send your prayers this way.”

Shelton said the city’s roughly 160 police officers and firefighters were going door-to-door, checking for people who might be trapped in the rubble. Search-and-rescue teams poured in from every corner of the state.

“This is terrible. This is war-zone terrible,” said a helicopter reporter for KFOR-TV (Channel 4) in Oklahoma City. “This school is completely gone. . . . This whole area is destroyed. The houses are destroyed, completely leveled.”

Oklahoma City tornado

The tornado outbreak was part of an explosion of violent weather that slashed the nation’s midsection, inciting severe-weather alerts from Texas to Michigan. The atmospheric conditions include a powerful weather system from the west colliding with warm, moist air from the Gulf of Mexico, a perfect recipe for tornadoes and super-cell thunderstorms, Schneider said. The bad weather will migrate to the east Tuesday.

The Storm Prediction Center counted nine tornadoes Monday in Oklahoma, Texas and Kansas, although that was a preliminary number based on witness reports. Although much of that region of the country is rural, one monster cell ground its way directly through the southern suburbs of the Oklahoma City metropolis — an urban hit that brought to mind the Tuscaloosa, Ala., tornado of April 27, 2011 (64 dead), and the Joplin, Mo., tornado on May 22 the same year (160 dead).

The strength of the big tornado that hit Moore will be determined in coming days after closer scrutiny of the damage. The twister was on the ground for 40 minutes, ravaging a 20-mile path, Schneider said.

“This was a very unfortunate path for this powerful storm,” he said. He noted that the likelihood of severe weather and tornadoes had been forecast many days in advance and that tornado watches had preceded the twister. But this was so powerful a storm, with such devastating winds, that people needed to be in basements or, better yet, a storm shelter, to be safe.

“You really need to be below ground,” Schneider said.

The residents there had seen this kind of thing before, on May 3, 1999, when more than 40 people were killed by a tornado that struck Moore, Newcastle, Del City, Stroud and other suburbs. That tornado reportedly spawned winds that reached 318 mph, the highest ever scientifically recorded, according to a 2005 story in USA Today.

“It’s as bad as it looks,” Rep. James Lankford (R), whose district includes most of neighboring Oklahoma City, said Monday evening as he left the House floor, checking his phone for updates.

The House will pause Tuesday for a moment of silence. On Monday night, President Obama spoke with Fallin to share his concern and assure the governor that the administration is ready to assist the state’s emergency responders. Late Monday, the president declared a major federal disaster and ordered federal aid to supplement state and local recovery efforts.

The injured were taken to multiple hospitals in the area, including 20 people, eight of them children, to Oklahoma University Medical Center, said spokesman Scott Coppenbarger.

About 60 people were taken to Norman Regional Hospital and Norman HealthPlex Hospital, said Kelly Wells, spokeswoman for Norman Regional Health Care. She described the injuries as “a lot of trauma, a lot of lacerations, a lot of broken bones. Typical tornado injuries. We’re no stranger to this out here.”

The two hospitals also received more than 30 patients from Moore Medical Center, the third hospital in Norman Regional’s network, which was destroyed by the tornado, Wells said.

By Monday evening, a Facebook page created to help connect survivors with loved ones had a growing number of posts, most from people searching for the missing.

“Looking for my Aunt Iris Irwin,” read one post.

“Looking for 5yo Harry,” read another.

Lenny Bernstein, Brady Dennis, Darryl Fears and Jason Samenow contributed to this report.