Warning: This story contains graphic details that may disturb some readers.
Four young men - freezing cold, starving and struggling to survive - stood over their dead friend armed with razor-blades and broken glass.
They cut away their friend's clothes. Then his body.
"I will never forget that first incision nine days after the crash," Roberto Canessa, who survived the 1972 Andes plane crash, wrote in his new book, "I had to Survive," according to an adaptation in the Daily Mail.
"We laid the thin strips of frozen flesh aside on a piece of sheet metal," he wrote. "Each of us finally consumed our piece when we could bear to."
Surrounded by death, they made the decision to live.
"Each of us came to our own decision in our own time," he wrote. "And once we had done so, it was irreversible. It was our final goodbye to innocence."
It was a Friday the 13th in October, 1972, when an Uruguayan aircraft carrying the "Old Christians" rugby team, and their friends and family, went down in the Andes in Argentina, near the border with Chile. After two months, 16 survivors were rescued - and became the inspiration for numerous documentaries, movies and books, most notably the 1993 film Alive, which was based on a book by the same name.
Canessa's own account comes out March 1.
In his book, Canessa remembered haunting moments - the one when the plane began to plummet and he held on to his seat with such strength that "I tore off chunks of fabric with my bare hands." The one when an avalanche buried him, and his friend started "frantically digging handfuls of snow away from my mouth." Or the one when they heard over their transistor radio that the search for them had ended.
But it seems it was the descent into cannibalism that was the hardest to endure.
In Canessa's adaptation in the Daily Mail, he wrote:
"Our common goal was to survive - but what we lacked was food. We had long since run out of the meagre pickings we'd found on the plane, and there was no vegetation or animal life to be found. After just a few days we were feeling the sensation of our own bodies consuming themselves just to remain alive. Before long we would become too weak to recover from starvation.
"We knew the answer, but it was too terrible to contemplate.
"The bodies of our friends and teammates, preserved outside in the snow and ice, contained vital, life-giving protein that could help us survive. But could we do it?
"For a long time we agonised. I went out in the snow and prayed to God for guidance. Without His consent, I felt I would be violating the memory of my friends; that I would be stealing their souls.
"We wondered whether we were going mad even to contemplate such a thing. Had we turned into brute savages? Or was this the only sane thing to do? Truly, we were pushing the limits of our fear."
Of the 45 passengers on the plane, 27 survived the crash. Then, one night, which Canessa called "the worst of my life," an avalanche killed eight more.
"We had no food - even the frozen bodies we were relying on to stay alive had been swept away," he wrote, according to the Daily Mail. "Everyone was waiting for someone to do something. Or for no one to do anything and just let the end come.
"That's when I steeled myself to do what needed to be done: to use one of the bodies of the newly dead."
Canessa wrote that he had "already done things that I never in my darkest nightmares imagined I'd have to do" - and he knew he had to do them again.
"And so we took yet another step in the descent towards our ultimate indignity: to eat the body of the person lying next to us," he wrote. "Each of us would have to be stained with this blood if we were to keep the seed of life from withering."
After a gut-wrenching 72 days on their own, 16 survivors were finally rescued on December 23, 1972.
But Canessa said he agonised over what they had done and how others would feel about it.
He recently talked to People magazine about seeing his mother and father.
"I told her, 'Mother, we had to eat our dead friends,'" he told People, "and she said, 'That's okay, that's okay, sweetie.' "
Canessa said he told his father that his main concern was how the victims' families would react to the harsh reality.
"I said, 'I don't care," he told People, '"the only thing I want to do is go to the families of my friends who died and tell them what happened. I don't expect them to understand but they should know what happened.'
"But thank God, people were very receptive and very supportive and they consider what we did something we had to do so everything went very smoothly."
Canessa, now a pediatric cardiologist, said it was his family - and the determination to make it home to them - that gave him the strength to survive.
"In these kinds of situations," he told People, "it's not how you survive but why you survive."