Illinois State Police say the bus blew a tire before veering off the road. NBC's Brian Williams reports.

Updated at 11:29 p.m. ET: A passenger bus traveling from Chicago to St. Louis plowed into a bridge support Thursday on a southern Illinois interstate, killing one person and injuring 33 others, several of them critically, police said.

Thirty ambulances and five emergency helicopters responded to the scene on Interstate 55 a few miles north of Litchfield, in Montgomery County south of the capital, Springfield, said Illinois State Trooper Doug Francis.

Illinois State Police told NBC station KSDK-TV of St. Louis that a woman seated above the driver on the second floor of the bus was killed. KSDK identified her as 25-year-old Aditi Avhad of India. She was on her way to Columbia, Missouri, the station reported. State police weren't able to provide precise information of the seriousness of people's injuries, saying in a statement only that "several" were critically injured.

The driver of the bus was taken to a hospital for surgery, but his injuries were not believed to be life-threatening, a statement from the Illinois State Police said.

The bus, a double-decker express operated by the budget intercity service Megabus, was running from Chicago to St. Louis, the company told NBC News. It said 81 tickets had been sold for the trip, but police said the bus' manifest showed only 64 passengers. The discrepancy couldn't immediately be explained.

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Police said they believed a tire blew out before the bus rammed into the concrete pillar. Video from KSDK showed that it sustained extensive damage.

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