Illinois State Police Trooper Doug Francis said the passenger who died was female, but he would not disclose her name or age. He did not know where she was seated on the bus, which was traveling between Chicago and Kansas City.
Francis said 38 people were taken to hospitals for injuries from the crash, which left the bus sitting with its crumpled front end smashed up against the bridge support. Rescue crews climbed ladders to reach those trapped inside, while others tended to injuries along the side of Interstate 55.
"There was a lot of screaming and crying," said 16-year-old passenger Baysha Collins, of Minneapolis, who was traveling to St. Louis to visit relatives. "There was blood everywhere. I was just in shock."
Megabus spokeswoman Amanda Byers said the bus was at full capacity, carrying 81 passengers, when it crashed near Litchfield, about 55 miles northeast of St. Louis. It left from Chicago and was to stop in St. Louis and Columbia, Mo., before arriving in Kansas City.
"We don't know what happened," Francis said. "Somebody reported to us it was a blown tire, but we haven't confirmed that yet."
The trooper said 33 people were taken by ambulance to hospitals, two were flown by helicopter to St. Louis hospitals and three were flown by helicopter to a hospital in Springfield, Ill. He did not know their conditions.
Memorial Medical Center spokesman Michael Leathers said late Thursday that seven people were being treated at the hospital, but he declined to reveal their conditions.
Paula Endress, spokeswoman for St. Francis Hospital in Litchfield, said 22 people were brought to the hospital for treatment, with two of them admitted due to bone fractures. She said none of the injuries were life threatening.
In St. Louis, Barnes Jewish Hospital spokeswoman Liz Kalicak said two patients, one in serious, the other in fair condition, were being treated. A 24-year-old man was being treated for multiple fractures at Saint Louis University Hospital. The hospital spokeswoman would not release his condition.
School buses took about 36 passengers from the crash site to the community center in Litchfield, said Janis Johns, transportation director of Litchfield Community Unit School District 12. The passengers were either uninjured or mildly injured and included some children, Johns said.
Collins, who was among those at the community center, was on the upper deck of the Megabus resting when, "all of a sudden, I heard a big boom. It felt like the wheel was skidding. It felt like the bus was going to tip over."
The teenager said that when the bus struck the pillar, she flipped out of her seat and landed on a stairway leading to the lower deck. Collins said she could hear people in the front of the bus moaning and crying.
By evening, many of the uninjured passengers already had been taken by bus from the community center to St. Louis. Others were picked up by relatives, including 27-year-old Megan Arns of St. Charles, Mo., a St. Louis suburb. Her parents made the 70-mile trip to get her.
Arns was on the top deck of the bus near the back talking to a woman next to her when "all of a sudden it felt like the bus ran over something really, really big." She said she could feel the bus lose control as it rolled into the median and toward the pillar.
"Absolute panic. People were screaming," said Arns, who got away with just a scrape on her head.
Arns and 22-year-old Enrique Villaroel of Chicago said passengers began helping each other almost immediately after the wreck.
"Panic at first, then total calm," Villaroel said. "Some people were carrying other people off the bus."
Villaroel said he also was on the upper level of the bus sleeping when he was awakened by screams. "I flew out of my seat and a little girl flew past me," he said, adding that the child appeared to be OK and he escaped with a few bruises.
A string of crashes involving low-fare buses in recent years have prompted calls for tougher regulation. Four passengers were killed in September 2010 when the driver of a double-decker Megabus smashed into a low bridge outside downtown Syracuse, N.Y. The driver was acquitted earlier this year of homicide in the deaths.
Fifteen people were killed in May 2011 when a bus swerved off Interstate 95 in New York City and was sliced in two. Two days later, another bus drove off the New Jersey Turnpike and struck a bridge support, killing the driver and passenger.
Federal Motor Carrier Safety Administration records show that Megabus did better than the national average on inspections and in safety rankings during the 24-month period that ended Wednesday. Megabus had three other crashes in 2011 in which one person died in each wreck, according to federal records. No other details were immediately available.
Since its launch in 2006, Megabus has expanded to more than 80 cities, serving more than 19 million passengers, the company says on its website. Megabus uses curbside stops to save money instead of building its own terminals and offers free Wi-Fi on the buses.
Megabus said in a statement that it is working with law enforcement authorities to investigate the cause of Thursday's crash.
"Safety remains our number one priority," the statement said. "The thoughts and prayers of our entire staff go out to the passengers involved."
The National Highway Traffic Safety Administration said in statements that it was aware of the accident and would work with local authorities "to determine if there are safety implications that merit agency action," but that the agency was not investigating the crash.
A strong thunderstorm rolled through the area about four hours after the crash. Francis said the rain did not complicate the rescue and recovery effort, although it did make the crash reconstruction more difficult.
Associated Press writers Carla K. Johnson, Caryn Rousseau and Sophia Tareen in Chicago and Danny Robbins in Dallas contributed to this report.