Investigators on Sunday combed through a patch of grass near an interstate highway, searching for clues to why a tour bus drifted off the pavement and overturned, killing 14 people.

Fifteen passengers remained hospitalized in Little Rock and Memphis, Tenn., many in critical condition with injuries that included collapsed lungs, broken hips and head wounds they suffered when the bus wandered off Interstate 55 in northeastern Arkansas (search).

A reconstruction of the accident was under way, and officials planned to create computer models of the crash, but they cautioned that a final police report would not be ready for a week. Findings from the National Transportation Safety Board (search) report would take longer.

"There is not going to be a magic answer provided to you today to the question, 'How did this crash happen?'" Arkansas State Police (search) spokesman Bill Sadler said.

Authorities said the probe would include determining whether the driver fell asleep, as well as reviewing the mechanical condition of the bus and whether it conformed to federal regulations.

They also want to know if weather or road conditions contributed to the wreck and why the bus' roof tore off during impact.

Police initially reported that 15 people had died. Investigators worked through the night to identify the victims and on Sunday revised that number to 14 dead. Most passengers were asleep when the accident happened before dawn.

On Sunday, the wreckage had been removed from the side of the highway. All that remained at the crash site was yellow police tape. State troopers were working to catalog passengers' personal belongings that had littered the ground Saturday at the scene.

Police said they interviewed two to three motorists who were driving behind the bus when the crash happened, as well as some survivors.

"After the trauma and shock and they come to themselves, they may remember more," state police Lt. Tommy Wicker said.

Gary Van Etten, an investigator for the National Transportation Safety Board, said authorities would look at the driver's experience and training.

Driver Herbert Walters, 67, the brother of the bus company owner, died in the crash.

Also killed were Sandra Clark, 62, John Hawkins, 49, Fannie Jacobs, 68, Charlie B. Lowe, 75, Maxie Lyons, 64, Mamie McCormick, 69, Genise Meekins, 50, William Morris, 58, Derrick Poole, 37, Cornelia Roseborough, 76, Joyce Drain, 64, Euzelia Vaughn, 57, and Walters' sister-in-law Mareen Walters, 67.

Van Etten said regulations for passenger-carrying operations prohibit drivers from driving more than 10 hours in a 24-hour period. Walters' family said the bus left Chicago at 8:30 p.m. Friday and the accident occurred at 5 a.m. Saturday -- a period of 81/2 hours.

When the crash occurred, the bus was less than an hour from its destination of Tunica, Miss., and the trip would appear to have been legal trip for Walters.

Van Etten said investigators would also look at the driver's diet and any medication he may have been taking. State police said Walters held the proper licenses to drive the bus.

"Is there an issue here that we can see something went wrong with the system from a safety perspective?" Van Etten said. "As tragic as this is, we look to make things better later on."

Marion Fire Chief Woody Wheeless said the roof was nearly completely torn off the bus before the vehicle landed upside down. One person was trapped under the bus and did not survive. Several other people were trapped under the roof, and emergency workers had to cut off part of the roof to reach them.

The speed limit for the driver was 65 mph. It was not known how fast Walters was driving. The driver's brother, Horace Walters of Little Rock, has said the bus' maximum speed was mechanically set at 62 mph.

Seat belts were not required on the bus.