The driver of a tour bus that crashed near a national park in Utah, killing four Chinese tourists and injuring dozens more, was making his first trip for a company that only been in business for a short time, investigators said Sunday.

The National Transportation Safety Board is researching the driver's background, license qualification and medical history, said Pete Kotowski, an investigator-in-charge for the agency, at a news conference Sunday night. He didn't disclose the driver's his name, but said he's from California. The agency has not yet interviewed him.

The driver is U.S. citizen and he didn't appear to be intoxicated, the Utah Highway Patrol said previously.

The NTSB is also researching the bus company's inspection history, hiring practices and corporate safety culture, Kotowski said. The company, called America Shengjia Inc., based in Ontario, California, is cooperating with investigators, he said.

He said it had been in business for a "short period of time," but didn't say exactly how long.

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Multiple phone messages left with the company Saturday and Sunday by The Associated Press were not returned.

The cause of the Friday crash near Bryce Canyon National Park in southern Utah has yet to be determined, Kotowski said. They're examining the crash scene as well as the mid-sized 2017 bus.

Utah Highway Patrol officials previously said it appeared the driver swerved and overcorrected, sending the bus rolling over into a guardrail near a highway rest stop a few miles from the national park. The crash left the top of a white bus smashed in and one side peeling away as the vehicle came to rest mostly off the side of the road against a sign for restrooms.

Four people died and five others were in critical condition as of Saturday night. Hospital officials didn't provide condition updates Sunday.

The bus had seat belts, but it's unknown if any passengers were wearing them, Kotowski said. Utah Highway Patrol Sgt. Nick Street said previously not everyone was wearing a seatbelt, as is common in tour buses.

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.

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The deceased were part of a 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. The tour group was dispatching employees from China to help those injured.

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.

The Shanghai Municipal Bureau of Culture and Tourism urged the travel agency, Shanghai Zhuyuan International Travel Agency, to spare no effort in rescuing the injured and properly handle the follow-up matters.

Phone calls to the travel agency rang unanswered Sunday morning. Lu Yong, the travel agency's general manager, told a Chinese TV program that the agency's American partners sent 10 staff members to hospitals to help the victims communicate with doctors and police.

The News Perspective program, part of the Shanghai Media Group, said in an article on its official social media account that seven relatives of the victims were expected to leave for the United States on Monday or Tuesday with travel agency staff and officials from the culture and tourism bureau.

The news program's social media post included photos of parts of the itinerary, indicating the accident occurred on the seventh day of a 16-day trip and also included visits to Yellowstone National Park, Salt Lake City and Las Vegas. They were 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.