PARK CITY, UTAH — August 1, 2008, was a beautiful day on K2, the 8,611-meter mountain on the border between China and Pakistan. Climbers from all over the world (including Serbia, Norway, Ireland, Korea, and the U.S.) had spent weeks camped on the mountain waiting for a weather window like the one that appeared that day for their chance to summit the world's second highest peak.

By day's end, 11 of those climbers were dead — and the ones who survived had widely divergent accounts of the events that transpired. The Summit, which premiered Friday at Sundance, grapples with the contradictory reports of what happened that day on K2.

"K2 is not a commercial peak. K2 is a mountain for professional mountaineers," Pemba Gyalje Sherpa said in an interview with BuzzFeed Entertainment on Sunday. Pemba was the climbing partner to one of the mountaineers who lost his life, Gerard McDonnell, and the film relies heavily on both his recollection and photographs he took that day when attempting to reconstruct the events.

Pemba and McDonnell met while climbing Everest in 2003. But unlike Everest, K2 requires a degree of technical skill and experience that makes it accessible only to the most elite class of climber — and the climbers' level of skill and degree of determination only increase as they make their way up the mountain.

There were 200 people at K2 base camp, but only 24 at Camp 4 (the last stop before the summit) on Aug. 1. When the weather cleared, almost every climber at Camp 4 was determined to summit.

Pemba, then 34, reached the peak with McDonnell, but made it back to camp safely. He would later venture back up the mountain to rescue Marco Confortola, an Italian mountaineer who descended with McDonnell. McDonnell himself was missing when Pemba found Confortola.

When news of the tragedy first began to emerge, director Nick Ryan said, "There were a lot of stories in the press about Wilco [van Rooijen] and Marco [Confortola] surviving," but, he added, "Pemba's contribution on the mountain had almost been written out."

Headlines reduced the 11 deaths to "inexperienced climbers unprepared for expedition," but according to the film, a fuller story began to materialize when McDonnell's family flew to Pakistan to speak with the survivors in hopes of understanding what happened to him. (The film suggests he died attempting to rescue two members of the Korean team, whose bodies Pemba discovered tangled together in ropes — but many details are still missing. McDonnell's body has never been recovered.)

According to Ryan, the biggest problem may have been the fact that there were too many capable climbers bent on making it to the top all at once. The night prior to the climb, "they are in this meeting and they say, if we're all going to go together we've got to work as one team. We can't work as individuals in this situation," Ryan said. In a group, he added, "we are sharing responsibility, and when you share responsibility, you think someone else is covering your ass."

Pemba agreed. "Two hundred persons in base, it's OK, but above the base camp when we reach Camp 1, when reach Camp 2, when we gain altitude day-by-day, when we reach 8,000 meters..." Ryan finished Pemba's sentence: "Everyone forgets everything."

"Those guys are bringing the rope. Where's the 200 meters the Italians had? Oh, I didn't bring it; I thought you were bringing it."

The film attributes the loss of several lives to logistical miscommunications like this. The morning of the expedition, for instance, the climbers' start was delayed because, according to the film, the leader of the Korean team did not set the fixed rope lines as others believed he agreed to do at a meeting the day before. (Of the five Korean team members who made it to the top of K2 on Aug. 1, only one is still alive. According to Ryan, that man repeatedly refused requests for an interview.)

The hold-up meant a number of climbers were forced to descend the summit in complete darkness. The darker and colder it got, and the more time the climbers spent with little oxygen, the harder it was for those climbers to hang on, and several of them perished.

As much as everyone — the press, the family, the filmmakers — wants to make sense of the tragedy, not every life that was lost that day on K2 can be traced back to a single event. A number of the deaths defy logic; they can only be written off as tragic accidents, like when a member of the Serbian team, eager to get to the top, unclipped his ropes and attempted to pass Norwegian Cecilie Skog — and fell to his death. Or, later, when Skog's husband, Rolf Bae, mere meters ahead of her, was hit and carried away by an icefall.

It is the random, unexpected turns of event like those that almost give The Summit the air of a horror film, said the film's writer, Mark Monroe, whose past work includes documentaries The Cove and The Tilman Story. "I think of, like, 10 Little Indians: The mountain is just slowly, one at a time, taking people, and you can't stop it."