Four people were killed in a tour bus crash outside Utah’s Bryce Canyon National Park that left at least a dozen others with critical injuries, the state’s highway patrol said Friday.

Thirty-one people, including the driver, were on the bus Friday morning when it went off Route 12, rolled once and landed on a guardrail, authorities said. Five people remained in “pretty critical condition” Friday evening, Utah Highway Patrol Cpl. Chris Bishop said.

Others were in stable condition with injuries ranging from “very minor to serious,” the highway patrol said via Twitter. Earlier in the day, it said 10 people had sustained “minor to serious injuries” in addition to 12 to 15 people with critical injuries. The passengers were all Chinese nationals, and the driver — who was hurt — is Chinese American.

Investigations so far indicate that the driver “for whatever reason drifted off the road” and then steered too hard to the left trying to correct course, Bishop said. The vehicle, which had been headed eastbound toward Bryce Canyon, rolled but ended up wheels-down when it hit the guardrail.

Those killed died on the scene, he added.

The crash occurred 10 miles east of the small town of Panguitch, according to the highway patrol. Authorities received calls about the crash just after 11:30 a.m.

“This is pretty overwhelming for a little county of 4,900 people,” Garfield County Commissioner Leland Pollock told local news media. “This is just horrible for us, and we feel terrible for those who are injured and their families.”

Intermountain Healthcare’s facilities received 22 patients, and five others were flown elsewhere for care, said spokesman Lance Madigan, adding that he could not yet describe the conditions of all of the patients.

Nineteen passengers were taken to Intermountain’s Garfield Memorial Hospital in Panguitch for treatment, Madigan said. At least 14 of those people were in critical or serious condition, the hospital said. Three patients were taken to Intermountain’s Sevier Valley Hospital in Richfield.

The National Transportation Safety Board is sending a 10-member team to arrive at the site Saturday to investigate the crash, the agency said on Twitter.

Bishop said officials expect to wrap up their on-scene work Friday evening, but reconstructing what happened could take weeks or months, he said.

Authorities will pull data from the vehicle’s equipment, such as speed, and are examining where the bus came from, the driver’s activities and whether he was impaired, Bishop said.

Route 12 was closed after the crash, KSL news reported.

In a tweet, the Chinese Embassy in Washington expressed sadness over the crash and thanked authorities in Utah for their response.

“The Embassy has initiated its emergency protocols, sent personnel to the area, and will assist the victims as needed,” it said.

Utah Highway Patrol said it is working with the Embassy to notify bus passengers’ families in China.

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