Federal investigators say they have not yet determined the cause of a tour bus crash in east Arkansas that claimed 14 lives. But the National Transportation Safety Board says the bus had pre-existing cracks on frame rails that held up its motor. The NTSB says the bus would not have been allowed on the road had it been inspected before the Saturday crash, which also left 16 people injured. The agency did not link the cracks to the crash. Investigators are still evaluating the bus, which was bound from Chicago to Tunica, Mississippi. The investigators say they will subpoena the Chicago medical records of the driver and will make inquiries in Tunica to uncover more background.