"We are focusing on the driver possibly being distracted for some unknown reason," said Arizona Department of Public Safety spokesman Lt. James Warriner.
He did not elaborate on what the distraction might have been.
Three survivors of Friday's crash near Hoover Dam have been interviewed, Warriner said.
Investigators have been unable to speak to the 48-year-old hospitalized driver of the bus because of his injuries. They are checking his driving history, medical records and certification to operate the bus, Warriner said.
The driver, 48-year-old Han Dong of Rosemead, Calif., remains in fair condition at University Medical Center in Las Vegas. Investigators said they don't believe he was impaired at the time of the crash.
The tour guide and six Chinese tourists were killed in the crash. Ten others were injured.
The California Public Utilities Commission on Monday suspended the charter certificate of D.W. Tour & Charter of San Gabriel, Calif., the company that owned the tour bus.