In 2013, dozens of people in Kalachi and Krasnogorsky, towns in remote Kazakhstan, began succumbing to a mysterious “sleeping sickness.” They started passing out at random—at school, at home, on a motorcycle—sometimes for days at a time. Kazakhstani officials announced last week they had an explanation: carbon monoxide drifting out of a nearby uranium mine.

Problem solved, right? Only...maybe not.

The Soviet-era uranium mine, abandoned since the early 1990s, was indeed an early suspect. But its most obvious associated dangers—like radon gas and radiation—didn’t fit the symptoms. Victims who didn’t pass out suffered hallucinations, headaches, and fatigue, while radon and radiation are more likely to increase cancer risk in the long-term. Scientists descended on Kalachi, a tiny 600-person village, and eventually ruled out a pathogen or man-made toxin as the cause. Theories about mass psychosis and even the supernatural took hold. Now, the official explanation is carbon monoxide.

It seems possible. “The symptoms fit,” says Claude Piantadosi, a pulmonologist at Duke University Medical Center. Carbon monoxide binds to human blood 200 times more strongly than oxygen, which means that it doesn’t take that much carbon monoxide to asphyxiate someone—even if there’s still oxygen in the air. A brain running low on oxygen begins to shut down, causing unconsciousness and other symptoms. “But the symptoms are not specific ,” says Piantadosi, “and that’s the problem.”

The gas is indeed a problem in coal mines—it’s what the proverbial canary dies from—but carbon monoxide is the byproduct of combustion. The uranium mine next to the two villages was inactive. So what was carbon monoxide doing in there? Perhaps the mine is more active, chemically or geologically, than usual.

Mine experts have other questions, too. Like, how did so much gas manage to escape into the open air at once? “I’m having a hard time with this,” says Robert Ferriter, a mine safety specialist at the Colorado School of Mines.

It’s easy for dangerous gases to get trapped in a closed space, like a mine with poor ventilation, according to Ferriter. But news reports note people in Kalachi have fallen asleep while riding a motorcycle, milking cows, and selling at a market stall. Gas escaping from a mine would have to travel a long way without dispersing, staying at a high enough concentration to cause problems above ground.

The answer shouldn’t be too hard to get; carbon monoxide poisoning shows up with a blood test. But back in January, Kazakhstan’s own health ministry reported that people who had fallen ill tested negative for carbon monoxide poisoning. (Though, granted, the test can be wrong if people get it too late.)

Piantadosi suggests other gases may have contributed to the phenomenon, like carbon dioxide, which is a natural component of our atmosphere and harder to detect. The government announcement last week did note hydrocarbons—likely methane gas—were also coming out of the mine. And, yes, some of the cases could also still have a psychological component.

In any case, officials aren’t taking any chances with a gas-belching uranium mine. The government is relocating villagers from Kalachi and Krasnogorsky to a safer location. Krasnogorsky was once a mining town of thousands; since the mine closed, its population has dwindled to just over a hundred. Now, thanks to the same mine, it may disappear forever.