WILLOW BEACH, Ariz. (AP) — A tour bus careened off the road and crashed in northwest Arizona late Friday, killing the driver and leaving at least four 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 most seriously hurt were flown by helicopter to hospitals, while other passengers were taken by ground ambulance.

Six were flown to University Medical Center in Las Vegas, where they were being treated for nonlife-threatening injuries, hospital spokeswoman Danita Cohen said.

She described them as being mostly over 50 years old and suffering from broken bones and lacerations.

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 four 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.