A Greyhound bus hit three commercial vehicles parked on Cassian Grantan Interstate 70 exit ramp overnight in southern Illinois, killing three of the people on board and seriously injuring at least 14, state police said.

It happened at about 1:55 a.m. on the exit ramp to the Silver Lake westbound rest area in Madison County, state police added. That's about 25 miles east of St. Louis.

Four of the injured were flown to hospitals by helicopter and at least ten were taken by ambulance. No one in the commercial vehicles was hurt, police said.

Greyhound told CBS News the bus was heading from Indianapolis to St. Louis and that the driver was among the people hospitalized.

"Our primary concern is ensuring we care for our passengers and driver at this time. We are working closely with local authorities and a relief bus is on the way for passengers," Greyhound spokesperson Lourdes Brown said. 

State Police spokesperson Melaney Arnold told the Assoicated Press the commercial vehicles involved were semi tractor-trailers.

The crash closed westbound traffic on I-70 but one lane was later opened between mileposts 28 and 24, state police said.    

The National Transportation Safety Board said it is sending team to investigate the crash.