FAIRBANKS (AP) — Michael Hopper and Erik Peterson heard warning sounds as they skied toward mountains in the eastern Alaska Range, but they thought they would be safe at higher elevations.

As they crossed a gently sloping valley in relatively flat terrain on Saturday they heard “whomping,” the sound of a layer of snow collapsing. They concluded it was snow settling into a creek bed.

They heard it again later in steep terrain, just before an avalanche killed Peterson, 35, and buried Hopper, 63, for more than two hours.

Hopper said he feels betrayed by the mountains he loves and thought he knew them better from trips along the same route, the Fairbanks Daily News-Miner reported.

“I may have gotten a little too confident, a little too familiar,” he said. “Maybe things are changing in ways none of us expected.”

Hopper is co-owner of the nearby Black Rapids Lodge. He has skied in the mountains for 20 years.

Peterson was a former assistant football coach at Delta Junction and West High in Anchorage and a former assistant track coach at Dimond High.

They had previous outdoor adventures together, including an attempt to climb Mount Hayes.

The first whomping sound they heard in the foothills Saturday was a little unnerving, Hopper said, but not enough to make them stop.

At the higher elevation, on an estimated 30-degree grade, they spotted another danger sign: Hard pack snow beneath six inches of loose snow. It was evidence of a weak layer that could move.

They changed course and headed for a west-facing ridge but heard another “whomp.” Peterson was ahead of Hopper and turned around to ask Hopper if he heard it.

“As he said that, I looked up-slope about 20 feet,” Hopper said. “I saw what looked like a wave breaking like whitewater. It just instantaneously appeared on the slope horizon.”

They had set off an avalanche about 600 feet above them. It carried Hopper 150 feet down the slope. When he stopped, his face and right arm were free but the rest of his body was encased in snow.

He scraped snow away with his free arm, loosened his left arm and dug himself out after two hours.

Peterson had landed about 10 feet up the slope. The avalanche covered his head and he could not breathe. His body has not been recovered because the mountain remains unstable.

Hopper dug out his skis and descended to the Richardson Highway, where a passing motorist assisted him.

Peterson’s dog, Rowdy, a Labrador-spaniel mix, disappeared in the avalanche and is presumed dead.