"We know he was distracted by that door," National Transportation Safety Board lead investigator Peter Kotowski told reporters. Witnesses told investigators the driver's side door was open at one point just before the bus crashed.
Kotowski said the driver also "indicated he was blinded by sun glare" just before he lost control of the bus around 4 p.m. on Jan. 30 as he was driving the group of Chinese tourists to Las Vegas from the Grand Canyon.
Investigators have not found anything wrong with the driver-side door or any other mechanical problems on the bus, Kotowski said. A determination on the cause of the crash won't be made until next year.
A tour guide and six Chinese tourists were killed in the crash along U.S. 93 in Arizona, 26 miles south of the Nevada state line. Ten others were injured, including the driver. Han Dong, 48, of Rosemead, Calif., remained hospitalized in fair condition Saturday.
Officials said they believe the driver veered onto the right shoulder and overcorrected before losing control, crossing a desert median and overturning. Investigators have not determined the speed of the bus at the time of the crash.