MOUNT RAINIER NATIONAL PARK, Wash. — Two Seattle-based climbing guides and four clients set out Monday to summit Mount Rainier in Washington state, following one of the most technical and physically grueling routes to the peak.

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

Park officials believe the group fell 3,300 feet from their last known whereabouts at 12,800 feet on Liberty Ridge. They are presumed to be dead in one of the worst alpine accidents on the mountain in decades.

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.

"We will likely fly over something this week if we have an aircraft," to monitor the situation, he said.

It's unclear whether the climbers were moving or camping at the time of the accident, park spokeswoman Patti Wold said. Searchers located camping and climbing gear and detected signals from avalanche beacons buried in the snow thousands of feet below their last known location.

"It's such a dynamic environment. You can have world-class climbers up there and they can't control the environment," Wold said.

It's also not known what caused the climbers to fall, whether it was rock fall or an avalanche.

Park officials said they expected to release the names of those who died later on Sunday. Meanwhile, family and friends of the dead climbers arrived at the mountain Sunday to meet with park officials.

"They're just devastated," Wold said.

Continuous ice fall and rock fall make the avalanche-prone area too dangerous for rescuers, and there are no immediate plans to recover the bodies, Wold said.

Like others who have died on the mountain, there's also a possibility they may never be found, park officials said.

The area will be checked periodically by air in the coming weeks and months, she said. They'll also evaluate the potential for a helicopter-based recovery as snow melts and conditions change.

Last year, about 10,800 people attempted to climb the 14,410-foot 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, the Disappointment Cleaver and Emmons.

Though the main climbing season for those popular routes is just starting, this is the season for the Liberty Ridge route, said Kessler, who is a former park climbing ranger.

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."

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

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. "You need to be in the best shape of your life," the brochure reads.

The guiding service lost five Nepalese guides in a deadly avalanche on Mount Everest in April. The avalanche that swept down a climbing route on the world's highest peak killed 16 Sherpa guides.

"It's devastating, it's emotionally draining, it's trying to make sense of it all," Janow said of the tragedies.

The loss of life on Rainier would be among the deadliest climbing accidents on the mountain. In 1981, 11 people were killed during a guided climb when they were struck by a massive ice fall on Rainier's Ingraham Glacier.

Le reported from Seattle. Associated Press writer Dinesh Ramde in Milwaukee contributed to this report.