SALT LAKE CITY (AP) — Several Chinese tourists injured in a deadly tour bus crash in Utah are improving, but three remain in critical condition, hospital officials said Monday.

The three people listed in critical condition marks a decrease from the five in that category on Saturday. The tour bus crashed in the red rock landscape of southern Utah on Friday, killing four Chinese tourists and leaving dozens more injured.

One other person is in serious condition and eight are in fair condition, Intermountain Healthcare officials said. They declined to provide any more information.

The bus with 30 adult passengers rolled over and slammed into a guard rail, crushing the top of the vehicle near Bryce Canyon National Park. Everyone on board was hurt.

The cause of the crash hasn't been determined by U.S. investigators, but they have said the driver was making his first trip. National Transportation Safety Board investigators are looking into the California man's driver's background, license qualification and medical history.

He is a U.S. citizen from California, officials have said, but his name has not been released. He didn't appear to be intoxicated, the Utah Highway Patrol has said.

The small California-based bus company's inspection history, hiring practices and corporate safety culture are also under investigation, said Pete Kotowski, investigator-in-charge for National Transportation Safety Board.

A Chinese-based travel company overseeing the U.S. trip, called U-Tour Group in Shanghai, defended its licensing and the bus driver. The parent company said in a statement Monday that the driver was qualified, the bus was insured, and the travel agency was properly registered with China's Ministry of Culture and Tourism.

About 18 family members of people who were killed or injured were preparing to travel to the U.S., U-Tour said.

The bus had seat belts, but it's unknown if any passengers were wearing them, according to Kotowski.

The operator of the bus, Ontario, California-based America Shengjia Inc.. was cooperating with investigators examining what caused the mid-sized 2017 bus to crash.

The licensed company has two vehicles and two drivers, according to the Federal Motor Carrier Safety Administration. Records show no history of previous crashes involving the company founded in 2015.

The company has not responded to requests for comment.

Three women and one man died in the crash. The victims have been identified as Ling Geng, 68, Xiuyun Chen, 67, Zhang Caiyu, 62, and Zhongliang Qiu, 65, according to the Utah Highway Patrol.

The group of 29 tourists and one leader came from Shanghai and the provinces of Zhejiang, Jiangsu and Heilongjiang, according to a news report on the media website huanqiu.com.

The News Perspective program, part of the Shanghai Media Group posted photos of parts of the itinerary indicating the accident occurred on the seventh day of a 16-day trip that also included visits to Yellowstone National Park, Salt Lake City and Las Vegas. The group was scheduled to fly to the East Coast after the western U.S. stops.

Millions of people visit Utah's five national parks every year. More than half of visitors from China travel on tour buses, said Vicki Varela, managing director of Utah Office of Tourism.

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McDonald reported from Beijing. Associated Press writer Brady McCombs in Salt Lake City contributed to this story.