Fister said the boulders were from 2 to 10 feet in diameter.

"It's highly unusual," Fister said. "We never had any other fatalities on McKinley by rock fall."

Clint West, a 47-year-old American living in the United Kingdom, died about 90 minutes after being hit by one or more rocks, according to the National Park Service.

Mark Morford, 47, of Portland, Ore., broke his right thigh bone and a wrist, and Gerd Islei, a 56-year-old German citizen, suffered several broken ribs, a collapsed lung and a ruptured disc in his lower back, the agency said.

Morford and Islei were flown off the mountain early Monday and taken to Providence Alaska Medical Center in Anchorage. Both were in serious but stable condition, according to Caitlin Palmer, co-director of the Talkeetna-based Alaska Mountaineering School, the guiding operation for which West, Morford and Islei were clients.

Morford and Islei declined to be interviewed, Palmer said Monday afternoon from Providence, where she had accompanied the injured climbers.

The rest of the team was eating and resting in tents at the 7,200-foot camp, manager Lisa Roderick said by radio phone. Weather between Talkeetna and the Alaska Range was too poor for ski planes to fly them out, Roderick said.

West, Morford and Islei were part of a 12-person party that included three guides. All had reached McKinley's 20,320-foot summit on Saturday via the West Buttress, the most common route, said Colby Coombs, the Alaska Mountaineering School's other co-director.

After spending the night at the 17,200-foot high camp, the party members decided Sunday they were feeling strong and would try to descend all the way to base camp, Coombs said from his Talkeetna office.

"They wanted to crank all the way down," he said.

Two other climbing teams were descending before them, a three-person Alaska Mountaineering School party that was ahead by an hour and a half and another large party, led by guides of Mountain Trip, that was about 15 minutes ahead, Coombs said.

Coombs had spoken to his expedition's lead guide, Rob Gowler, on the mountain by cell phone and to McKinley rangers in Talkeetna, he said.

Morford, Islei, West and Mountaineering School guide Steve Grillo, in that order, were roped together, Coombs said. They were the first of three four-person rope teams on the descent.

The rock slide began about 9:30 p.m. without warning as the first rope team reached 13,200-foot Windy Corner, the National Park Service said.

Windy Corner is notorious for an icy slope falling away toward crevasses and an ice cliff that drops thousands of feet to a fork of Kahiltna Glacier.

Above the ice slope are the rock bands and gullies of the West Buttress. The common trail goes below and fairly close to the bands.

"Rock fall is something we're all aware of in that area," Coombs said. "The route hugs the buttress because all these crevasses are on the other side. Five years ago my head almost was taken off by a rock that was grapefruit-sized."

Sunday's event apparently was a landslide or a massive rock slide, Coombs said.

He and Fister both said they did not have a clear idea of how large the slide was. But witnesses told them lots of debris was evident. Moreover, to have hit both Morford and West, who had been spaced about 120 feet from each other on the rope, and Islei in between, the rocks had to have fanned out as they crossed the trail.

"Gerd (Islei) said this huge rock was bounding toward him and he thought he was going to die, but it ricocheted," Coombs said.

"Mark's pack looks like a train ran over it," he added. "The ski pole is snapped in two, and a plywood stove board in his pack was in multiple pieces. The shoulder strap of his pack is severed in half, cut right through the foam, like someone took a scissors and snipped the strap in the middle."

Gowler, the party's lead guide, described the largest rock that fell as the "size of my Ford Ranger," according to Coombs.

Coombs, author of a climber's guide to the West Buttress route, said it was likely that abundant sunlight had caused the rocks to spring loose.

Recent temperatures on the mountain were not unseasonably high, but skies had been clear for two days, according to Fister. The West Buttress slope above Windy Corner has a southern exposure and receives plenty of direct sunshine.

Other guides on McKinley told Coombs "there was a surprising amount of water that they heard coming around the corner, indicating significant melting" up the slope, he said.

"The word on the trail is that things are hot, temperaturewise, and who knows if there was finally enough heat to melt enough ice for this to become unstable enough to let go," he said.

"While small rocks and snow shed regularly at Windy Corner during warmer weather, the climbers were descending at night when cooler temperatures and frozen conditions make for safer travel," the Park Service said in a written statement.

"It was freakish and unlucky," Coombs said of the slide and its timing.

West is the fourth mountaineer traveling with a permitted guide service to die on McKinley since 1971, according to Park Service records. A licensed guide died there in 1992.

Altogether, 93 men and women have died climbing McKinley and its glaciers, about half while descending, the records indicate. All but three died in the past 37 years.