An eyewitness says 25 climbers reached the summit on Friday, but nine were stranded and froze to death after an avalanche swept away their fixed ropes.
In the deadliest day in K2's history, another climber fell to his death and a porter died recovering a body. The avalanche happened when a chunk from an ice pillar snapped away on a feature called the Bottleneck.
Swedish climber Fredrick Streng, who had decided not to attempt the summit, gave the BBC details of the events. He said several expeditions, with a total of 25 climbers, had taken advantage of Friday's fine weather to reach the summit of K2. But he said they returned late, and were tired when the avalanche struck out the fixed lines.
"There were still nine climbers above these ropes, which meant they had to spend the whole night out in the open," he said. "The next morning they were still there, not moving. And people during the night had been hit by the avalanche. One had died, one had fallen at an earlier stage.
"At the end of Saturday, 11 people were confirmed dead."