FORT CHIPEWYAN, Alta. -- RCMP and Parks Canada are now treating the search for four missing hunters in northeastern Alberta as a recovery operation after failing to find the men.

RCMP said searchers were using boats equipped with sonar Wednesday to search beneath the surface of the Rocher River north of Fort Chipewyan.

"Our search has now moved to a recovery operation," RCMP Cpl. Ronald Bumbry said.

"These men have not been located on land. We don't want to speculate on what occurred but we have now moved our search and recovery efforts to below the water and the surrounding banks to rule out all possibilities."

The hunters left the community on Sunday night in a boat which was later found in the river that flows through Wood Buffalo National Park.

More than 70 people have been searching the remote, heavily wooded area including a crew from Fort McMurray, a dog team and three helicopters.

Bumbry said the families of the missing men have been notified.

The hunters are experienced outdoorsmen.

The searchers have included volunteers from the Mikisew Cree and the Athabasca Chipewyan First Nations.

"Our thoughts are with family members," he said. "Some are assisting in the search and recovery efforts."

Temperatures in the area have been above freezing since Sunday.

The river, partly filled with ice, was flowing swiftly, Bumbry said.

No names have been released.