Steadily falling snow blanketed the Midwest on Thursday, part of a vicious cold snap engulfing much of the nation, and a tractor-trailer spun out of control on snow-slick roads in Ohio, killing four people and injuring seven.

The tractor-trailer jackknifed on Interstate 70, crossed the highway median and swerved into oncoming traffic, colliding with a small bus transporting adult disabled passengers, the Ohio Highway Patrol said.