The details are sketchy. The incident occurred on May 22 on Tibet’s North Col route up Everest, according to a British climber who witnessed it. The climber in question, a Han Chinese whose name we still don’t know, drew suspicion when he camped apart from the other two Chinese expeditions on the mountain. If you’re trying to poach Everest—that is, climb it without a permit—keeping to yourself and camping away from other groups is a dead giveaway.

As with climbing from the more popular south (Nepalese) side of the mountain, permits on the Chinese side of Everest represent only a small portion of the total cost—$25,000 on the low end. Even so, the risk of getting caught, having to pay a large fine, and forfeiting the expense and effort that goes into the rest of the trip usually dissuades people from attempting to climb it illegally. Plus, the vast majority of climbers enlist the services of professional guides, who wouldn’t even consider signing up an unpermitted climber. Still, it does happen.

But how exactly a group of graduates of the Tibet Mountaineering Guide School (TMGS), who were working as rope fixers on behalf of all the other commercial expeditions, figured out that he was climbing without a permit, and why they decided to do what they did, remains unclear. (Attempts to reach officials at TMGS have thus far gone unanswered). The incident started at 25,500 feet. The TMGS grads confronted the Chinese climber, who wielded his ice axe as a weapon. They subdued him, bound his hands, and marched him down to the North Col at 23,000 feet.

The British climber, who asked not to be named, described the incident in an email to Billi Bierling, the assistant of Everest historian Elizabeth Hawley. It read in part:

“I did see the permitless chap being ushered down the hill.  The Tibetan rope fixers were sent up to get him. I saw them bringing him down the ropes from the North Col to [advanced base camp]. It was disgraceful. They literally kicked him down the ropes. It was a disgusting example of a pack of bullies egging each other on and literally beating him down the hill. It was absolutely unnecessary as he was offering no resistance and was scared out of his mind.  The Tibetans should, and could, have just escorted him down the hill and let the authorities deal with him.”

Kari Kobler, 57, a Swiss guide and the proprietor of one of the world’s largest outfitting operations, Kobler & Partner, also encountered the procession. Kobler has climbed Everest five times and has been guiding the mountain since 2000. He filmed the brutal scene playing out in front of him but hasn’t made the footage public. Though he wouldn’t let me watch the video when I met with him over lunch in Kathmandu, he did briefly describe it after I mentioned that I’d heard rumors about it.

The men dropped their captive, “like a rucksack with an oxygen tank, but actually it is a human being,” says Kobler, who believes that the incident could inflame tensions in the region or give the Chinese authorities an excuse to further restrict Tibetan freedom. “It’s a tough one. It’s really tough. I know all of them.”

“The reason I made this video—If I don’t have proof, nobody will believe me. [The TMGS grads] can lie and say nothing happened. But now I can go to them and say, ‘Please, young boys.’