The wintry weather was expected to continue through midweek, and ice storm warnings stretched from Texas to Pennsylvania.
"Tomorrow may be even more of a dilemma than today because we're going to get even a little bit more colder," said John Pike, a meteorologist in the Weather Service's office in Norman, Oklahoma, on Sunday.
Six traffic deaths were blamed on icy roads in Oklahoma .
More than 130,000 customers lost power in Missouri, Oklahoma, Illinois and Kansas, utilities reported.
Missouri Gov. Matt Blunt declared a state of emergency and activated the National Guard.
Chicago's O'Hare International Airport, one of the country's busiest, canceled more than 400 flights. The airports in Kansas City, Missouri, and St. Louis also canceled several flights.