Three women and one man perished in the crash on a highway running through the red-rock landscape of southern Utah on Friday.

The victims were identified as Ling Geng, 68, Xiuyun Chen, 67, Zhang Caiyu, 62, and Zhongliang Caiyu, 65. They were all from Shanghai, China.

They were part of a tour group made up of 29 tourists and one leader. They come from Shanghai and the nearby provinces of Zhejiang, Jiangsu and Heilongjiang, according to a news report on the media website huanqiu.com.

The tour leader came from Hebei Province, near Beijing, according to the Zhejiang Online news site.

Five passengers remained in critical condition Friday night, and the death toll could rise, Utah Highway Patrol Sergeant. Nick Street said.

All 31 people on board were hurt. Twelve to 15 on board were considered to be in critical condition shortly after the crash, but several of them have since improved, Street said.

Not everyone was wearing a seatbelt, as is common in tour buses, he said.

The crash happened near a highway rest stop a few miles from southern Utah's Bryce Canyon National Park, an otherworldly landscape of narrow red-rock spires.

Authorities believe the driver swerved on the way to the park on Friday morning. But when he yanked the steering wheel to put the bus back onto the road, the momentum sent the bus into a rollover crash, authorities said.

The driver, an American citizen, survived and was talking with investigators, Street said.

He didn't appear to be intoxicated, but authorities were still investigating his condition as well as any possible mechanical problems, he said.

The National Transportation Safety Board was sending a team to investigate.

Australian Associated Press