A dump truck on Tuesday slammed into a tour bus on a busy Chicago interstate, injuring more than a dozen people, many of them senior citizens, officials said.

Seventeen people were taken to hospitals, at least one of them in serious condition, Chicago Fire Department spokesman Larry Langford said. He said 11 other people declined treatment.

Most of the injured bus riders were just shaken up, Langford said. Emergency crews used backboards and neck braces as they took several people to waiting ambulances.

The dump truck rear-ended the bus after the bus driver hit the brakes because of slow traffic due to an accident in the road ahead, officials said. Earlier, officials had said the bus had hit a car first.

The bus had a "Horseshoe Casino Hammond" designation and was being used by Chicago Entertainment Tours, though it wasn't immediately clear if the Indiana casino was on the group's itinerary, officials said. A message left with the company was not immediately returned.

The accidents shut down the northbound lanes of Interstate 57 on the city's south side for over an hour during the morning rush.