WILLOW BEACH, Ariz. — A charter bus carrying tourists back from the Grand Canyon careened off a northwest Arizona highway late Friday night and crashed, killing the driver and injuring more than 20 passengers, six of them seriously, authorities said.

The bus was northbound on Highway 93 and nearing the Nevada state line when the driver suffered a physical problem and lost control shortly before 8 p.m., the Arizona highway patrol said.

The vehicle hit a ravine, tore up a small hill, and then bounced and lurched over rough terrain for more than 700 feet before stopping, damaging the front end but remaining upright, patrol spokesman Carrick R. Cook said.

“We believe the driver experienced some sort of the medical condition and he just went off the road,” the patrol said in a statement. No other vehicles were involved.

All 48 passengers were injured, but 27 of them were walking wounded with mainly bumps and bruises, Cook said.