The driver of the tour bus that crashed in Utah late last week, killing four on board, was taking his very first trip for a company that had only been in business for a short time, investigators said.

The doomed bus crashed Friday near Bryce Canyon National Park in southern Utah, killing three women and one man. The Utah Highway Patrol has identified the victims as Chinese tourists Ling Geng, 68, Xiuyun Chen, 67, Zhang Caiyu, 62, and Zhongliang Qiu, 65.

The National Transportation Safety Board is probing the incident, researching the driver’s background, license qualification and medical history, according to Pete Kotowski, an investigator-in-charge for the agency.

Kotowski did not identify the driver, but said he is a US citizen from California and did not appear to be intoxicated. He has not yet been interviewed.

The Ontario, California-based tour bus company, America Shengjia Inc., is cooperating with investigators, according to Kotowski.

It has been in business for a “short period of time,” according to Kotowski, but he didn’t specify exactly how long.

The NTSB is also looking into the company’s inspection history, hiring practices and corporate safety culture, Kotowski said. The company did not return calls from the Associated Press.

Highway Patrol officials initially said it appeared the driver swerved and overcorrected, which sent the white bus careening into a guardrail near a rest stop a few miles from the national park.

All of the 31 people on board were injured, five of whom were listed in critical condition, according to the last update Saturday night.

There were seat belts on the bus, but it’s not clear whether any of the passengers were wearing them. It’s common on tour buses for passengers to skip the seat belt, Highway Patrol Sgt. Nick Street said.

The crash happened on the seventh day of the group’s 16-day trip that also included West Coast stops at Yellowstone National Park, Salt Lake City and Las Vegas. Then they were set to fly to the East Coast for the second leg of their trip.

With Post wires