WASHINGTON, Sept.22 (Xinhua) -- The U.S. bus crash that killed four Chinese tourists last weekend took place during the driver's first trip, the National Transportation Safety Board (NTSB) said Sunday.

This Sept 20, 2019 photo shows the bus crash scene near the Bryce Canyon National Park in Utah, the United States. (Photo: Xinhua)

"We do know that this driver was recently hired, and that this was his first trip," Pete Kotowski, who led the NTSB investigation team, told a press conference.

The driver has returned to his home in California after being released from the scene of the crash that happened Friday near the Bryce Canyon National Park in Utah. Four Chinese tourists were killed in the accident and more than 20 others injured.

The identity of the driver is not released. An earlier statement by the Utah Highway Patrol said he is a Chinese American.

Kotowski said police have obtained the driver's blood sample to determine if he was under influence at the time of the crash.

According to Kotowski, the NTSB will further question the driver. It will look into the tour company's hiring practices, the driver's licensing and medical conditions.

Local media cited official records as saying the crashed bus was registered to America Shengjia Inc. in California. Reports said the company has two drivers and no records of crash or failed safety inspections in the last two years.

Kotowski said a full investigation into the crash may take 12-24 months.

Preliminary investigation shows the bus drifted off the road to the right, and the driver overcorrected to the left and the bus rolled over before slamming into a guardrail.