The weather service estimated that the tornado was at least a half-mile wide and says it could have been on the ground for as long as 40 minutes.
Moore resident Melissa Newton said the hail from the tornado was "about the size of golf balls."
As Moore continues to sift through rubble for survivors, millions across the Midwest are once again under the threat of tornadoes. People in northeast Texas all the way to southwest Arkansas have a 10 percent chance of seeing a twister later today.
Millions of people from San Antonio, Texas, all the way to Michigan could see damaging hail and even a chance of isolated tornadoes.
More than 50 tornadoes ravaged the Midwest this weekend, killing a 79-year-old man in Shawnee, Okla.
Monday's devastation in Oklahoma came almost exactly two years after an enormous twister ripped through the city of Joplin, Mo., killing 158 people and injuring hundreds more.
Joplin City Manager Mark Rohr said his community remembers the assistance it received in 2011 and believes it has an obligation to lend a hand in Moore.
Moore was the site of one of the most destructive tornadoes in U.S. history. An EF-5 tornado ripped through the Oklahoma City-area May 3, 1999, killing 42 people.