A tour bus that crashed along an Arkansas highway, killing 14, was in such poor condition before the wreck that it would have been pulled out of service had it been inspected, investigators said Wednesday. The National Transportation Safety Board said it found pre-existing cracks on frame rails that held up the motor in the rear of the bus. The agency did not link the cracks to the crash, saying the cause remained under investigation. Authorities were still looking at whether the driver, Herbert Walters, fell asleep at the wheel and planned to subpoena his medical records. The cracks should have been discovered during an annual inspection and resulted in an "out-of-service" order, which would have required the company to fix the problem before the bus was allowed back on the road, said Gary Van Etten, the NTSB's lead investigator. Authorities do not know when the bus was last inspected. "We were told it was done just before this trip, but what just before this trip means, I don't know," Van Etten said. The bus crashed along Interstate 55 about 25 miles north of Memphis, Tenn., when it failed to follow a left curve, careened off the roadway and flipped, killing Walters and 13 passengers. The accident happened in a light mist shortly before dawn. Authorities also discovered that sheet metal had been glued and riveted to the roof, adding 600 to 700 pounds to the bus' weight. Investigators would not say whether they believe the heavier roof affected the crash. Investigators have said 29 of the 30 people on the bus were thrown from the vehicle when much of the roof peeled away. Sixteen people were injured, six of whom were still being treated Wednesday at hospitals in Little Rock and Memphis. The 1988 motorcoach was sold in 1995 or 1996 to Walters Bus Service Inc. of Chicago, which is owned by Roosevelt Walters, the brother of the bus driver. Investigators have not determined when the roof was altered. At least one wrongful death lawsuit has been filed in the accident.