Marion, Ark. ? The lone bus belonging to a mom-and-pop tour operator careened off an interstate and overturned early Saturday, killing 15 Chicago-area travelers on their way to a Mississippi casino. Witnesses told police the bus, which carried family and friends of the tour company owner, was drifting.

The bus was about 30 miles short of its destination in Tunica, Miss., when the crash happened about 5 a.m. on Interstate 55 in northeastern Arkansas, near Memphis, Tenn.

Thirty-one people were aboard, and the remaining 16 passengers were injured, many critically. Some of the dead were found crushed beneath the bus after crews pulled it upright, state police spokesman Bill Sadler said.

Among the dead was the bus owner’s brother, who had been driving. The owner’s wife, stepson and niece also died. The driver, Herbert Walters, was believed to be in his 60s.

Witnesses told police the bus drifted off the road near a point where the interstate veers toward Memphis.

The bus, “just kind of faded over there,” said Cpl. Mickey Strayhorn of the Arkansas State Police. “There was not really any erratic driving before this occurred.”

A light mist was falling at the time of the crash, but visibility did not appear to be significantly limited, police said.

The impact tore off a section of the bus’ roof, and emergency workers had to shear off the rest of the top to reach a trapped passenger.

Tracks in the grass showed the bus went straight rather than around the curve, then hit a ditch and flipped over. There were no skid marks.

Emergency personnel remove a victim from the scene of a bus crash in Marion, Ark. The driver of the tour bus and 15 passengers were killed in the crash early Saturday. Police say the bus was carrying 31 Chicago tourists to a casino in Tunica, Miss., when it overturned along Interstate 55.

“They had to cut it (the roof) free from the bus, but it was peeled off more or less,” Strayhorn said.

Other victims, thrown from the bus, were scattered among grass at an interstate exchange, along with popped-open suitcases and other belongings.

Sgt. David Moore, describing the force of the impact, said it would be “similar to an explosion. There were people everywhere.”

Assistant Fire Chief John Burns of West Memphis said when he and a crew of firefighters arrived at the scene, “there was nobody walking around — everybody was laying down.

“It wasn’t the scene where you see everybody screaming and crying for help,” Burns said.

State police had a partial list of passengers and were going through debris to identify the others.

Sadler said 14 people died at the scene. Hospitals reported receiving a total of 17 people, including one who later died.

Sandy Snell, spokeswoman for The Medical Center at Memphis, said that hospital was treating eight victims, including three in very critical condition, three in critical condition, and two with serious injuries.

Police cautioned that investigators may not know what happened for weeks.