In an era of dwindling “firsts” of merit, it is rare that mountaineers climb on to the front pages. There came an exception on January 17 when dozens of newspapers around the world carried images of a Nepalese former Gurkha.

Nirmal “Nims” Purja, who lives in Hampshire, had defied the odds and the sceptics the day before by completing the first winter ascent of K2, the world’s second-highest and perhaps most fearsome mountain. Until then, the 8,611m peak had stood alone among the world’s 14 summits that rise above 8,000m, the only one never to have been climbed in winter, when winds and temperatures are even more vicious.

Moreover, Purja’s feat was a triumph shared. Ten Nepalis, who had arrived on the mountain as members of three separate teams but united during the ascent, held hands and sung their national anthem as they stepped on to the summit together. After risking their lives in service to western expeditions for a century, often without much reward or credit, Nepalese climbers had bagged what some billed as the last great prize in high-altitude mountaineering.

“History made for mankind, History made for Nepal!” Purja’s Instagram feed announced as the men descended, before returning to Kathmandu and a heroes’ welcome. It was, Purja would say, “bigger than the World Cup”.

Yet less than an hour after their triumph, K2 began to show why it is known as the “Savage Mountain”, confirming fears in the climbing community about the unprecedented mass assault on such a forbidding peak in the coldest months. Whereas previous winter attempts had been undertaken by a handful of climbers in a single team, this year saw more than 50 hopeful summiteers converge on the peak, a mix of professionals and those who had paid to join a commercial expedition. Tragedy soon engulfed the mountain, this time away from the front pages, reviving debates about risk, ego and the commercial pressures of adventure in the modern age.

As Purja and his colleagues were starting to descend the upper slopes, Sergi Mingote, a vastly experienced Spanish mountaineer, fell to his death. We may never know what caused the accident, somewhere between Camp 1 and advanced base camp. He would not be the last of K2’s victims.

Storms largely confined the remaining climbers to base camp for the next two weeks. History had already been made, but any disappointment at missing out on the “first” mingled with a thrilling realisation. “We were like: ‘Woah, this can be done . . . Now we can do it, too,’” recalls Colin O’Brady, an American adventurer. They played Jenga, tried to keep fit, and forged fragile friendships.

Time was running out when a tantalising window appeared in weather forecasts. Climbers mobilised on February 3. There were two groups — a crack unit of professionals who had missed out on the earlier summit window, and a big commercial team.

The pro team was led by John Snorri, 47, a father of six and Iceland’s best known mountaineer. He was joined by Ali Sadpara, 45, a Pakistani climber who had risen from poverty and service as a porter to become a national hero and mountaineering great — among his numerous accomplishments was the first winter ascent of the 8,126m Nanga Parbat, in 2016. With them was Ali’s 22-year-old son Sajid, who, like his father, had already climbed K2 in the summer.

O’Brady was part of the bigger team of about 40 climbers, half of them Sherpas, who had come with Seven Summit Treks. The Nepalese company had defended itself against criticism for taking paying clients of relatively mixed ability on such an expedition. “There are always certain and uncertain risks, especially in K2 in the winter,” Chhang Dawa Sherpa, the expedition manager at Seven Summit Treks, told the FT in December. “I just can say we will try our best.”

The climbers progressed in smaller groups. Some leapfrogged Camp 1, one of the four main possible resting spots on the ascent. They all planned to skip Camp 4, which would make Camp 3 (7,350m) a vital launch pad for the 12 to 15-hour summit push, where climbers could rest for a few hours, melt snow for water, eat and change their socks.

O’Brady woke up at Camp 2 on February 4 to worrying new weather reports. Winds on the summit looked like they were going to get stronger earlier the following day. A narrow weather window was shrinking. He hurried to pack for the ascent to Camp 3. “I set off at about 9.30am on a really beautiful, sunny day with no wind,” he says two weeks later from his home in Wyoming.

K2: the route to the summit map

O’Brady made short work of the Black Pyramid, a dangerously exposed section of steep rock and ice. But the going was hard, and then the fixed ropes ended just below Camp 3. They may have become buried by windblown snow. Climbers had to navigate crevasses without ropes, further delaying their arrival at Camp 3.

What followed remains somewhat cloudy, but it was immediately clear that everyone was running late, and that there was a severe shortage of tents. About 20 people attempted to huddle into three or four tiny tents in temperatures of -35C, making it impossible to properly rest ahead of a summit bid. Other tents that had been cached weeks earlier had blown away.

“Climbers were forced to pile into tents like sardines in a can,” recalled Elia Saikaly, a Canadian film-maker who was working with Snorri’s team, in a blog published 10 days later. He turned back from Camp 3 when he could not find the oxygen bottles that were supposed to have been stashed for him. His chance to film the summit attempt over, he retreated to a lower camp and listened to radio chatter up at crowded Camp 3.

“Why the fuck aren’t there more tents up here?” Saikaly says one climber shouted to another over the radio. Someone else said: “I can’t feel my toes up here, I haven’t changed my clothes and I’m freezing.” The film-maker also recalled hearing a climber saying of another who was determined to keep going up: “That guy is going to kill himself.”

By about midnight, seven climbers had gone for the summit. The others, including O’Brady and Noel Hanna, the only UK climber on the peak, decided to descend at dawn. “I had to draw the line between summit fever and wanting to come home,” says Hanna, who is from Northern Ireland. Three of those who had ventured out returned about two hours later with frostbite and reports of a large crevasse, requiring a long detour.

High risk: fatality rates on the world’s highest peaks

This left four climbers above Camp 3: Ali and Sajid Sadpara, Snorri and Juan Pablo “JP” Mohr, 33, a Chilean father of three and experienced climber on the Seven Summits team. Sajid later turned back. He had made it with the others to the notorious Bottleneck couloir, about 400m from the summit, when he tried to turn on his supplementary oxygen system only to find it was not working. Extreme cold also knocked out Snorri’s and Mohr’s GPS trackers, as well as radio and satellite phone batteries. The men were alone.

Hanna and O’Brady began their descent in the morning with the three climbers who had swiftly aborted their summit push. The danger was far from over. New ropes are often installed alongside old ones, which can weaken and fray over time or disappear under deep snow. Old anchors can become loose. Choosing the right rope on which to clip oneself can be a life-or-death decision. “Sometimes you’ll clip into old and new ropes, because there’s nothing to say even the new rope will hold you,” Hanna says. “A new anchor could pull out.”

Hanna was descending the steep Black Pyramid when, below him, Atanas Skatov, an accomplished Bulgarian climber, disappeared. Saikaly, who had joined the descent, was just below Skatov. “A body flew directly over my head out of nowhere,” he later wrote. “There was no warning. There were no cries. There was no sound.”

The climbers think Skatov might have made a fatal error while clipping from one rope to another. He fell for more than 1,000m. When the rest of the group got back down, they had to break the news to Sheny, the Bulgarian’s girlfriend, who had been staying with him at base camp.

Hanna arrived at base camp in the middle of the night. When he got up for breakfast to learn that nobody had heard from Snorri, Sadpara or Mohr, his fears multiplied. “They had been out walking for 32 hours and, even with the oxygen they had, that time in minus 50C or 60C? It’s a no-no,” he says. “It was hard because I’d got to know them really well.”

Sajid Sadpara stayed in Camp 3 long after everyone else had descended, desperately scanning the mountain above for the flash of his father’s head torch, or a glimmer of his brightly coloured down suit. It is hard to imagine the emotions surging through his mind when eventually he descended alone.

It wasn’t until February 18 that Snorri, Sadpara and Mohr were officially declared dead at a press conference in Pakistan, days after military aircraft searches had been called off. “My family and I have lost a kind-hearted person and the Pakistani nation has lost a brave and great adventurous individual who was passionate about the Pakistani flag to the point of insanity,” Sajid Sadpara said. He felt confident the three men had reached the summit, and that something had gone wrong on their way down.

As families in five countries mourn the loss of men who had more than a dozen children between them, an expedition post mortem goes on. The mood of this month’s press conference could not have been in starker contrast to the celebrations that greeted Purja and his countrymen in Kathmandu. Were the deaths an unavoidable consequence of testing human limits on the roof of the world? Or did they push too far?

“I’m the last person who will ever second-guess their decision-making,” says Alan Arnette, a mountaineer and chronicler of the sport who has himself climbed K2. “They made the best decision with the information available to them.” Nevertheless, the American, who followed every metre of the expedition, describes the combination of worsening weather and the conditions at Camp 3, which made rest and rehydration difficult, as “a recipe for disaster”.

As we wait for more detail to emerge, Arnette is relieved there were not more fatalities. He had been most concerned for the fate of relative novices, all of whom favoured caution. The fallen five were among the most experienced climbers on the mountain, with more than 35 8,000m summits between them. “And they’re the ones who ended up losing their lives,” he says. “That’s a testament to K2 and climbing 8,000m peaks in winter — it doesn't matter who you are, your life can be taken at the snap of a finger.”

Follow on Twitter to find out about our latest stories first