At least four people are dead after a tour bus rolled over on a Utah road, ejecting some passengers, authorities said.

Thirty people, including the driver, were on board when the bus rolled over near Bryce Canyon National Park on SR-12, Garfield County Sheriff’s Office spokeswoman Denise Dastrup said.

“Everyone on the bus sustained some sort of injury,” Dastrup said.

Garfield Memorial Hospital received 17 patients from the crash; officials said three were in critical condition, 11 in serious condition and three in fair condition.

There appeared to be “no obvious explanation for the rollover,” Garfield County Sheriff James Perkins told ABC News. He said the crash was on “part of the road that had a slight gradual turn,” but “not much of a turn.”

SR-12 is now closed in both directions, about 3.5 miles west of the park. The national park remains open.

ABC News’ Matthew Fuhrman contributed to this report.