The saiga antelope is a funny yet endearing looking creature with large dark eyes, a light tan coat and an odd floppy nose. It’s also critically endangered and only lives on the steppes of Central Asia. That makes the sudden deaths of more than half of the world’s population last May a tragic mystery that researchers are scrambling to solve.

Now they have pointed the finger toward the agents responsible for the die off, reports Carl Zimmer for The New York Times.

Work in the weeks immediately following the deaths and over the summer led scientists to believe that the saiga succumbed to a bacterial infection of the blood. Some kind of pathogen multiplied so rapidly in their bodies that toxins overwhelmed them. Testing continued through the early fall and now researchers have announced that the infectious agents are two bacteria, Pasteurella multocida and Clostridium perfringens.

Experts also updated the count of saiga killed from initial estimates of 120,000 to 211,000. That’s 88 percent of the saiga that visit Betpak-dala in Kazakhstan every spring, the calving grounds for the species’ largest population.

“I’ve worked in wildlife disease all my life, and I thought I’d seen some pretty grim things,” Richard A. Kock, of the Royal Veterinary College in London, tells The New York Times. “But this takes the biscuit.”

However, the case isn’t closed quite yet. The two pathogens named are normally harmless microbes that live in the respiratory systems and intestinal systems of many animals. For reasons the scientists don’t fully understand, they can occasionally turn deadly. But for the microbes to kill, the saiga must have had weakened immune systems.

A windy, cold spring could have done it, Zimmer reports, and this past year’s was unusually harsh. Climate change is a potential culprit on many experts minds.

The timing of the spring’s rough weather meant that the antelope were shedding their winter coats and also giving birth or nursing. All the factors stacked together could have been enough to cause the catastrophic number of deaths in a species already prone to such die offs.

The only thing humans can do is bolster the numbers of saiga still living to protect against future dangers. Representatives from Kazakhstan, Mongolia, Russia,Turkmenistan and Uzbekistan agreed in a meeting last week to improve captive breeding programs and look for ways to open and keep migration routes for the saiga. Otherwise, another mass die off could leave the saiga facing extinction.