A tour bus careened off the road and crashed in northwest Arizona late Friday night, killing the driver and leaving six passengers with serious injuries, authorities said.
About 45 other passengers were less seriously hurt and not all of them required hospital treatment, the Arizona Highway Patrol said.
The crash occurred shortly before 8 p.m. PDT on Highway 93 near Willow Beach, about 20 miles from the Nevada state line and 40 miles southeast of Las Vegas.
The bus was northbound when it went off the road and into a ravine. No other vehicles were involved.
"We believed the driver experienced some sort of the medical condition and he just went off the road," the patrol said.
The six most seriously hurt were flown by helicopter to University Medical Center in Las Vegas, while 15 others passengers with less severe injuries were rushed to area hospitals by road.
UMC spokewoman Danita Cohen said the six were being treated for nonlife-threatening injuries, such as broken bones and lacerations. She described them as being mostly over 50 years old.
The patrol statement said officers were trying to determine what happened to the bus after it went off the roadway. The front end was heavily damaged.
Four hours after the crash, the bus was sitting upright about 30 feet off the shoulder of the road.
It appeared to have at first gone down a ravine before traveling up an embankment.
The highway near the crash site was reduced to one lane of traffic.
A bus crash nearby nearly four years ago killed seven people.
Authorities said a charter bus carrying a group of Chinese tourists on a return trip to Las Vegas from the Grand Canyon on Jan. 30, 2009, crashed on U.S. 93 near Hoover Dam. Ten others were injured, including the driver.
AP writer Bob Seavey in Phoenix contributed to this story.