LITCHFIELD, Ill. — As a Megabus slammed into a bridge support pillar on an interstate highway in Illinois, the impact was so powerful that it flung 16-year-old passenger Baysha Collins from the upper-level seat where she was resting to a stairway leading to the lower level.

From there, she heard moaning from her fellow passengers on the double-decker bus, the front end of which was so mangled from the collision that emergency crews had to use ladders to rescue those trapped inside. At least one passenger was killed and dozens of others were hospitalized following the Thursday crash, authorities said.

“There was a lot of screaming and crying,” said Collins, of Minneapolis, who was on her way to St. Louis to visit relatives. “There was blood everywhere. I was just in shock.”

Aditi R. Avhad, 25, a native of India, was killed in the crash, Illinois State Police Trooper Brad Lemarr said late Thursday. Lemarr said she was headed to Columbia, Mo., but he didn’t know where she was currently living or from where she was traveling. Authorities also did not know where she was seated on the bus.

At least 38 people — nearly half of those on the bus — were taken to hospitals and at least five who were transported by helicopter, Trooper Doug Francis said.

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, Mo.

Collins, who was among the three-dozen passengers taken from a crash site to a community center in Litchfield, said she first heard a “big boom,” as if the wheel was skidding.

“It felt like the bus was going to tip over,” she said.

Francis said it wasn’t immediately clear what caused the crash and that he couldn’t confirm reports there was a blown tire. 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.

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.

Janis Johns, transportation director of Litchfield Community Unit School District 12, said the passengers brought to the community center were either uninjured or mildly injured and included some children.

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.

Francis 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.

Early Friday, some hospital officials said their staffs were still treating patients, though none were reporting any of the injuries to be life-threatening. They included Paula Endress, spokeswoman for St. Francis Hospital in Litchfield, who said many of the 22 people brought there for treatment had been released, though she did not know the actual number. In St. Louis, Barnes Jewish Hospital spokeswoman Liz Kalicak said two patients remained at the hospital early Friday, one in serious condition and another in fair condition. 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 the 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.”