Relatives of more than a dozen travelers who were hurt when their tour bus drifted off a highway kept vigil Sunday at hospitals while investigators sought a cause for the crash that killed 15 people.

Authorities did not know why the bus, traveling from Chicago to a Mississippi casino, crashed early Saturday off Interstate 55 (searchin northeastern Arkansas, injuring the remaining 16 passengers, who were family and friends of the tour operator.

On Sunday, the survivors remained at various hospitals in Tennessee and Arkansas, many in critical condition with injuries that included collapsed lungs, broken hips and head wounds.

One of those injured, Theophilus Cannon, was unable to speak to his sister, Octavia Eddings. But he wrote on a notepad, telling her "I feel better."

His fiancee, Shirley Fox, told Eddings she recalled feeling "a big bump" on the bus and saw Edding's brother go flying past her, tumbling.

"She saw another guy go to the left. She said it was an instant. There was no warning. Nothing," Eddings said.

Meanwhile, state police and investigators from the National Transportation Safety Board (searchmet Sunday behind closed doors to compare findings about the crash. A news conference was scheduled for midday.

The bus belonged to a mom-and-pop tour operator based in Chicago. The group left Chicago on Friday evening and had planned to spend the weekend at the casino.

Arkansas State Police spokesman Bill Sadler originally said 15 people were killed, then put the number at 14. It was not clear Sunday if state police were including in their count a person who was injured and later died at a hospital.

Among the dead was the bus driver, Herbert Walters, and tour organizer Mareen Walters. Herbert was the brother of bus owner, Roosevelt Walters. Mareen was Roosevelt's wife.