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.

Twelve to 15 bus passengers suffered “very critical injuries,” the highway patrol said.

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

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.

This is a developing story. Please check back for updates.