Rescuers likely know the final resting place of six climbers who set out last week to attempt one of the most technical and physically gruelling routes to the peak of Mount Rainier in Washington state.

But the danger of recovering the bodies of the two guides and four climbers believed to have fallen 3,300 feet (1,000 metres) from their last known location is too great, park officials say.

"People are very understanding that we cannot risk another life at this point," Patti Wold, a Mount Rainier National Park spokeswoman, said Sunday.

The climbers were last heard from at 6 p.m. local time Wednesday when the guides checked in with their Seattle-based company, Alpine Ascents International, by satellite phone. The group failed to return Friday as planned.

They are presumed dead in one of the worst alpine accidents on Rainier since 1981, when 11 people were struck and killed by a massive ice fall on the Ingraham Glacier.

Family and friends of the dead climbers arrived at the mountain Sunday to meet with park officials.

"They're just devastated," Wold said.

Beacon signals detected

It's unclear whether the climbers were moving or camping at the time of the accident, Wold said. Searchers located camping and climbing gear and detected signals from avalanche beacons buried in the snow at the top of the Carbon Glacier at 2,895 metres in elevation.

It's also not known what caused the climbers to fall from their last known whereabouts at 3,900 metres on Liberty Ridge, whether it was rock fall or an avalanche.

Glenn Kessler, the park's acting aviation manager, said "they are most likely buried," making recovery efforts even more challenging. They may be in an area too hazardous for rescuers to reach on the ground.

Missing Rainier Climbers

Mount Rainier National Park spokeswoman Patricia Wold said the danger of falling rock and ice in the area where searchers picked up pings from the climbers' avalanche beacons prevents a ground recovery effort. (Marcus Yam/The Seattle Times/Associated Press)

Continuous ice fall and rock fall make the avalanche-prone area too dangerous for rescuers, Wold said. The area will be checked periodically by air in the coming weeks and months, she said. They will also evaluate the potential for a helicopter-based recovery as snow melts and conditions change.

Wold initially said that the park on Sunday would release the names of the six who died but later said the park cannot release the names for privacy reasons.

Rob Mahaney told The Associated Press that his 26-year-old nephew, Mark Mahaney, of St. Paul, Minnesota, was among those presumed dead. He said the climber's father and brother flew to Seattle on Saturday after learning what happened.

Mahaney said his nephew had climbed Rainier before.

"He just loved to climb, he loved the outdoors, he loved the exhilaration of being in the wide open," Rob Mahaney said. "Even as a toddler he was always climbing out of his crib. His parents couldn't keep him anywhere — he'd always find a way to get out of anything."

Liberty Ridge route less travelled

Last year, about 10,800 people attempted to climb the 4.392-metre glaciated peak southeast of Seattle, but only 129 used the Liberty Ridge route, according to park statistics. The vast majority use two other popular routes.

Gordon Janow, director of programs for Alpine Ascents International, said the group was on a five-day climb of the Liberty Ridge route.

Mount Rainier Fall

An active volcano, Mount Rainier rises 4,392 metres above sea level and, along with Mount Baker, dominates the Seattle skyline. (Ted S. Warren/Associated Press)

The climbers had to meet certain prerequisites, and their ice and technical climbing skills as well as their biography were evaluated by a three-person team, Janow said.

The company's brochure says, at a minimum, those interested in the guided climb were required to be able to physically carry a 50-pound backpack on steep snow and icy slopes, ranging from 30 to 50 degrees in slope.

The guiding service lost five Nepalese guides in a deadly avalanche on Mount Everest in April that killed 16 Sherpa guides.