A line of thunderstorms battered Iowa with winds of up to 100 mph early Monday, knocking down trees and power lines and blacking out more than 200,000 homes and businesses.

No injuries were reported, and there were only a few reports of structural damage, including a roof torn off a small building at the state prison for women in Mitchellville near Des Moines.

The storms didn't produce a lot of rain, but a wind speed of 100 mph was reported at Dawson, a town of some 150 people about 30 miles northwest of Des Moines, the National Weather Service said.

By late Monday morning the storm system had crossed Illinois into Indiana, where the weather service posted severe thunderstorm warning for northern counties.

In Iowa, about 177,000 customers of MidAmerican Energy were without power from Sioux City on the state's western edge to the Davenport area on the east.

"It started about midnight in Sioux City. One of the unique things about this storm was that it never really broke up. It just moved across our entire service territory," said Ann Thelen, spokeswoman for MidAmerican Energy.

Thelen said the major problem was "an enormous number" of trees on power lines. She said she had no idea yet how long it would take to restore service.

Iowa's other large utility company, Alliant Energy, reported 31,000 customers without power.

Power outages in Des Moines shut down traffic lights, snarling the morning commute. Police posted signs or directed traffic at the busiest intersections.

At the women's prison, part of the roof hit cars in a parking lot but no one was hurt, said Fred Scaletta, spokesman for the Iowa Department of Corrections. He said the building houses a voluntary clothing donation center for inmates who are leaving prison.