A monstrous tornado at least a half-mile (800 meters) wide roared through the Oklahoma City suburbs Monday, flattening entire neighborhoods and destroying an elementary school with a direct blow as children and teachers huddled against winds up to 200 mph (320 kph). At least 51 people were killed, including at least 20 children, and officials said the death toll was expected to rise. The ferocious storm — less than 1 percent of all tornadoes reach such wind speed — ripped through the suburb of Moore in a Midwest region of the U.S. known as Tornado Alley. The storm laid waste to scores of buildings in Moore, a community of 41,000 people about 10 miles (16 kilometers) south of Oklahoma City. Block after block lay in ruins. Homes were crushed into piles of broken wood. Cars and trucks were left crumpled on the roadside. Rescuers launched a desperate rescue effort at the elementary school, pulling children from heaps of debris and carrying them to a triage center. The National Weather Service estimated that the tornado reached up to a half-mile wide and was an EF-4 on the enhanced five-point Fujita scale, the second most powerful type of twister. More than 120 people were being treated at hospitals, including about 50 children. And search-and-rescue efforts were to continue throughout the night. In video of the storm, the dark funnel cloud could be seen marching slowly across the green landscape. As it churned through the community, the twister scattered shards of wood, awnings and glass all over the streets. Oklahoma Gov. Mary Fallin deployed National Guard members to assist with rescue operations and activated extra highway patrol officers. Fallin also spoke with President Barack Obama, who declared a major disaster and ordered federal aid to supplement state and local recovery efforts. "Hearts are broken" for parents looking for their children, Fallin told a news conference. At Plaza Towers Elementary School, the storm tore off the roof, knocked down walls and turned the playground into a mass of twisted plastic and metal. Children from the school were among the dead, but several students were pulled alive from the rubble. Rescue workers passed the survivors down a human chain to the triage center in the parking lot. James Rushing, who lives across the street from the school, heard reports of the approaching twister and ran to the school, where his 5-year-old foster son, Aiden, attends classes. Rushing believed he would be safer there. "About two minutes after I got there, the school started coming apart," he said. The students were sent into the restroom. A man with a megaphone stood near a Catholic church Monday evening and called out the names of surviving children. Parents waited nearby, hoping to hear their sons' and daughters' names. Don Denton hadn't heard from his two sons since the tornado hit the town, but the man who has endured six back surgeries and walks with a severe limp said he walked about two miles (three kilometers) as he searched for them. As reports of the storm came in, Denton's 16-year-old texted him, telling him to call. "I was trying to call him, and I couldn't get through," Denton said. Eventually, Denton said, his sons spotted him in the crowd. They were fine, but upset to hear that their grandparents' home was destroyed. As dusk began to fall, heavy equipment was rolled up to the school, and emergency workers wearing yellow crawled among the ruins, searching for survivors. Because the ground was muddy, bulldozers and front-end loaders were getting stuck. Crews used jackhammers and sledgehammers to tear away concrete, and chunks were being thrown to the side as the workers dug. Many land lines to stricken areas were down, and cellphone networks were congested. The storm was so massive that it will take time to establish communications between rescuers and state officials, the governor said. Tiffany Thronesberry said she heard from her mother, Barbara Jarrell, shortly after the tornado struck. "I got a phone call from her screaming, 'Help! Help! I can't breathe. My house is on top of me!'" Thronesberry said. Thronesberry hurried to her mother's house, where first responders had already pulled her out. Her mother was hospitalized for treatment of cuts and bruises. The tornado also destroyed the community hospital and some retail stores. Moore Mayor Glenn Lewis watched it pass through from his jewelry shop. "All of my employees were in the vault," Lewis said. Chris Calvert saw the menacing tornado from about a mile (1.6 kilometers) away. "I was close enough to hear it," he said. "It was just a low roar, and you could see the debris, like pieces of shingles and insulation and stuff like that, rotating around it." Even though his subdivision is a mile (1.6 kilometers) from the tornado's path, it was still covered with debris. He found a picture of a small girl on Santa Claus' lap in his yard. A map provided by the National Weather Service showed that the storm began west of Newcastle and crossed the Canadian River into Oklahoma City's rural far southwestern side about 3 p.m. When it reached Moore, the twister cut a path through the center of town before lifting back into the sky at Lake Stanley Draper. Oklahoma City Police Capt. Dexter Nelson said downed power lines and open gas lines posed a risk in the aftermath of the system. Monday's powerful tornado loosely followed the path of a killer twister that slammed Moore in May 1999. That storm produced the highest winds ever recorded near the Earth's surface — 302 mph (486 kph). Kelsey Angle, a weather service meteorologist in Kansas City, Missouri, said it's unusual for two such powerful tornadoes to track roughly the same path. It was the fourth tornado to hit Moore since 1998. A twister also struck in 2003. Oklahoma City has had more tornado strikes than any other city in the United States," the city government's website says. Monday's devastation in Oklahoma came almost exactly two years after an enormous twister ripped through the city of Joplin, Missouri, killing 158 people and injuring hundreds more. On Monday, Joplin organized a team of about a dozen police and firefighters to assist in Moore. Joplin City Manager Mark Rohr said his community remembers the assistance it received in 2011 and feels an obligation to lend a hand in Moore. That May 22, 2011, tornado was the deadliest in the United States since modern tornado record keeping began in 1950, according to the National Oceanic and Atmospheric Administration. Before Joplin, the deadliest modern tornado was June 1953 in Flint, Michigan, when 116 people died. Country music star Toby Keith, who grew up in Moore, saidhis hometown would persevere. "Hometown got hit for the gazillionth time. Rise again Moore Oklahoma," Keith tweeted Monday evening.