Several vehicles spun out and the traffic behind them was unable to stop in time. In all, 46 vehicles, including 18 semis, were involved in the crash. Several cars were crushed between the trucks and some slid underneath them.



A witness told WGN-TV a line of brake lights suddenly appeared out of the snow.



"As soon as we [saw] the brake lights, already too late," he said. "So we looked back, told everybody, 'Brace yourself for impact.' "



His truck hit a semi and was then struck by at least three other vehicles, he said. "We had to climb out the back of our windows and the sunroof," he said.



Scott Collins, 17, of Chesterton, Ind., told reporters he was riding in a car with three other teens and saw the crash happen just behind them.



"One of the semis started sliding and I think it jackknifed in the middle of the road" and collided with another semi, he said. "After that happened, multiple semis locked up. . .We were pretty nervous."



Troopers and firefighters scrambled in the dark and cold, searching for victims. Helicopters were called in to rush the most seriously injured to hospitals.