A Mount Rainier National Park spokeswoman said camping gear had been found and distress signals heard from the group's avalanche beacons.
She said there was little chance for survival.
The group, from Seattle's Alpine Ascents International, was last heard from on Wednesday.
There were two guides and four clients in the party, which began a five-day expedition last Monday.
The park spokeswoman, Fawn Bauer, told Reuters the camping gear was found more than 3,000ft (900m) below the group's last known position and the group may have been caught in an avalanche.
"We don't believe there was a viable chance for survival," she said.
The search effort has been suspended.
Ms Bauer said: "It would expose our rangers to pretty extreme conditions, so we are not able to do any kind of ground searching of that area. And, in all honesty, we may never be able to get on the ground there."
The last contact was via satellite phone on Wednesday at an elevation of 12,800ft.
At that time a rain system was sweeping through.
When the party failed to return on Friday a helicopter was despatched.
Mount Rainier stands at 14,410ft and attracts thousands of climbers each year.
Hopes are fading for six climbers who have gone missing on Mount Rainier in the north-western US state of Washington.