A federal investigator Monday said the driver of a tour bus involved in a deadly crash apparently had been driving all night without backup.

"We need to look at hours of service," said National Transportation Safety Board (search) investigator Gary Van Etten. "What he did during the daytime hours would be of great interest to us so we could evaluate any possible fatigue that he might have had."

The crash Saturday along Interstate 55 north of Marion, Ark., killed driver Herbert Walters and 13 Chicago-area passengers on their semiannual trek to a Tunica, Miss., casino. The wreck occurred just before dawn, in a light mist, when the bus failed to follow a left-hand curve, left the roadway and flipped.

Sixteen survived the crash; two remained in critical condition Monday at the Regional Medical Center in Memphis, Tenn.

From her hospital bed, survivor Twanda Frazier told Chicago television stations about the aftermath.

"Every time I think about it ... I see dead people laying next to me. The paramedics are saying ... he's a goner, she's a goner. And I look to the left ... and he's dead, and then I look to the right ... and she's dead. And I'm raising my hand, oh, I'm not dead. I made it," said Frazier, who is recuperating from a broken neck, dislocated shoulder, cuts and bruises.

Investigators are examining the mangled bus and reconstruction could take as long as three months. They are trying to determine whether Walters, 67, lost control of the vehicle or if some mechanical failure caused it to go off the road with no signs of skidding or braking.

The NTSB said the bus' roof, which came off during the accident, also is an area of concern. Past NTSB documents have raised questions about bus integrity — with roof supports getting smaller as bus windows get larger.

Van Etten said other motorists on the highway told police they thought the bus was going about 70 mph. However, a brother of Walters, former Little Rock police captain Horace Walters, said the bus was mechanically set to go no faster than 62 mph.

The Federal Motor Coach Safety Administration (search) also is looking at the company, Herbert Walters' history and whether he had ever submitted to a required random drug test, agency spokesman Dave Longo said.

The bus is owned by Roosevelt Walters, whose wife Marean also died in the crash.

Thomas Price, attorney for Walters Bus Service Inc. of Chicago (search), said no lawsuits have been filed and the company is cooperating with authorities.

"It's a tragedy beyond tragedy. We offer our condolences to all of the families who have lost their lives and those who are injured," Price said.

The investigations may be as much about overall bus safety as the driver or bus company's record. Similar probes of past crashes have yielded safety recommendations that ended up ignored or rejected.

The NTSB recommended seat belts on buses several times, as far back as 1968, but the National Highway Traffic Safety Administration and the Federal Highway Administration have decided repeatedly not to implement the requirement.

The safety board argued most fatalities in rollovers occurred when the passenger was ejected from a seat or out the window. But studies also found lap belts alone could cause internal injuries and bus structures don't easily lend themselves to the shoulder-strap models now required in cars and trucks.

There were no seat belts for the passengers on the tour bus; Van Etten said the only seat belt that's required is for the driver. He didn't know if Walters was wearing one.

"He was fun-loving, joked around a lot. But when it comes to a bus, he's a serious person," brother Willie Walters said.