Six fatalities were confirmed on the scene, about 40 miles northwest of Kingman; a seventh person died at University Medical Center in Las Vegas, said Cmdr. Dean Nyhart with the Arizona Department of Public Safety.
Sgt. Tom Eaves, a DPS spokesman, said investigators from the National Transportation Safety Board will be arriving on the scene Saturday to investigate the deadly crash.
Members of the DPS vehicular crimes unit are also investigating the crash. State investigators plan to talk with crash survivors when they're able, Eaves said.
The tour bus was traveling north on U.S. 93 when it veered right and then left across the median. It rolled at least once before stopping, Nyhart said.
He said he believes all but one of the victims killed at the scene had been ejected from the bus. About half of those killed were women, he said.
Nyhart said five people had suffered life-threatening injuries. "We're hopeful that those people can be stabilized and at least the death toll stop where it is," he said.
The bus driver was among those in critical condition, he said.
Tourists on the bus were Chinese nationals who had flown from Shanghai to San Francisco and had most recently been in Las Vegas, according to DPS.
The group left Las Vegas early Friday for a trip to the Grand Canyon and was returning Friday afternoon when the crash occurred.
The bus was still resting on its side in the road late Friday, its front bumper torn in half and its rooftop emergency exit open.
Several bodies covered in tarps or blankets also remained in the roadway.
Witnesses said the crash area was strewn with money and papers from the victims.
According to DPS, the bus belonged to D.W. Tours out of San Gabriel, Calif. A recorded message on the company's phone line said the mailbox was full and that no messages could be left. The company didn't respond to an e-mail from The Associated Press seeking comment.
Nyhart said the bus could hold 24 passengers.
Several people rushed to the scene from a nearby diner after the crash.
Sheila Larsen, who works at Rosie's Den, said she went outside after hearing a big crash and screams and found people lying on the roadway.
"I will never forget, ever," said Larsen. "It's always going to haunt me."
She said the best she could do to comfort the victims was bring them blankets and towels and water. "The woman I was trying to comfort didn't speak English. I was trying to comfort her and she died," Larsen said.
Debbie Diamond, who also works at Rosie's Den, said she's seen many accidents on that highway but never one like that. "I just keep praying, that's all you can do in a situation like this," she said.
Hospital spokesman Rick Plummer said five people remained hospitalized at University Medical Center in Las Vegas, including the driver, a 61-year-old man in critical condition. The others were: a 35-year-old woman in serious condition, a 48-year-old man in serious condition, a woman in critical condition whose age was unknown and an 8-year-old boy in serious condition.
Ryan Kennedy, a spokesman for Kingman Regional Medical Center, said one victim, an adult male in critical condition, was transported to Sunrise Hospital in Las Vegas. The remaining four three women and one man were all listed in serious condition.