At first, the team suspected that a new infectious disease had spread through the population, but the pattern of deaths just didn’t fit. The saiga were dying too synchronously and too quickly. Also, all of them had died. “In biology, there’s certain rules, you know?” says Kock. “We accept that sometimes microbes can cause us harm, but not like this. Even very severe viral diseases or anthrax don’t do this. A good proportion of the animals would be fine.”

News of the die-off sparked outlandish explanations about Russian rocket fuel, radiation, and even aliens. But while conspiracy theories raged, a huge international team of scientists, led by Kock, got to work. Vets autopsied as many saigas as they could. Ecologists sampled the soil. Botanists checked the local plants. They couldn’t find any signs of toxins that might have killed the saiga. Instead, the actual culprit turned out to be a bacterium, one that’s usually harmless.

Pasteurella multocida normally lives in the saiga’s respiratory tract, but Kock’s team found that the microbe had found its way into the animals’ blood, and invaded their livers, kidneys, and spleens. Wherever it went, it produced toxins that destroyed the local cells, causing massive internal bleeding. Blood pooled around their organs, beneath their skin, and around their lungs. The saigas drowned in their own bodily fluids.

But that answer just led to more questions. Pasteurella is common and typically harmless part of the saiga’s microbiome. In livestock, it can cause disease when animals are stressed, as sometimes happens when they’re shipped over long distances in bad conditions. But it has never been linked to a mass die-off of the type that afflicted the saigas. What could have possibly turned this docile Jekyll into such a murderous Hyde?

The team considered a list of possible explanations that runs to 13 pages. They wondered if some environmental chemical or dietary change had set the microbe off. They checked if biting insects had transmitted a new infection that interacted with Pasteurella. They considered that Pasteurella might have gone rogue because of an accompanying viral infection, in the same way that Streptococcus bacteria can bloom during a cold, leading to strep throat. “We tested for everything and we couldn’t find anything,” says Eleanor Milner-Gulland from the University of Oxford.

Only one factor fit the bill: climate. The places where the saigas died in May 2015 were extremely warm and humid. In fact, humidity levels were the highest ever seen the region since records began in 1948. The same pattern held for two earlier, and much smaller, die-offs from 1981 and 1988. When the temperature gets really hot, and the air gets really wet, saiga die. Climate is the trigger, Pasteurella is the bullet.

It’s still unclear how heat and humidity turn Pasteurella into a killer, and the team is planning to sequence the bacterium’s genome to find out more. But for now, Kock says the connection makes sense. In laboratory studies, rats that are exposed to Pasteurella are more likely to get infected if you crank the humidity up. The idea of humidity as an environmental trigger also explains how the die-off happened so broadly, suddenly, and totally. It’s something pervasive, which would hit all the saiga at once, and influence the bacteria that they all already harbor.