Twenty-eight people were injured, three critically, when a Red Arrow passenger bus enroute to Edmonton from Fort McMurray rolled over Friday afternoon near Redwater, Alta.

Five passengers were trapped inside the fully-loaded bus and had to be extricated.

Alberta Health Services spokesman Kerry Williamson confirmed that 28 passengers were taken to hospitals in Edmonton, St. Albert, Redwater, Fort Saskatchewan, and Westlock; nine others were assessed at the scene but did not require ambulance transport.

Two of the critically injured people were airlifted to hospitals in Edmonton. The third was transported by ground ambulance.

At least 12 ambulances were dispatched to the scene, Williamson said.

RCMP Sgt. Jennifer Jackson said the victims' injuries ranged from minor cuts and bruises to the three more serious cases.

The rollover happened at 2:15 p.m. on Highway 28, about 10 kilometres west of Redwater, northeast of Edmonton.

Conditions on the road were dry and the weather was sunny when the bus flipped on the two-lane highway.

"The bus was fully loaded at the time of the collision and doing, presumably, highway speeds," Jackson said.

Officials with Red Arrow bus lines were stunned by the incident.

"Shaken-up is probably a good word," said general manager John Stepovy. "We've got professionals on site here and we're just trying to take care of the passengers as best we can here."

Highway 28 was closed in both directions while police continued their investigation.