In May, 60,000 saigas -- a critically endangered species of antelopes found in the Eurasian steppes -- dropped dead in a span of just four days in central Kazakhstan, baffling scientists, conservationists and veterinarians alike. Soon, more than 120,000 saigas -- nearly half of the global population of the antelope -- died, making it one of the most significant mass death events among the species.

“I have worked in veterinary diseases all my career and I have never seen 100 percent mortality [within a herd],” Richard Kock, a wildlife veterinarian at the Royal Veterinary College in the U.K., told Nature in June. “That is extraordinary.”

Saigas -- characterized by a distinctive bulbous nose -- are found in herds in Kazakhstan, Mongolia and Russia. The animals, which once numbered in the millions, are prone to massive, and unexplained, die-offs.

However, “the extent of this die-off, and the speed it had, by spreading throughout the whole calving herd and killing all the animals, this has not been observed for any other species,” geoecologist Steffen Zuther, international coordinator for Kazakhstan's Altyn Dala Conservation Initiative, told LiveScience Friday. “It’s really unheard of.”

Scientists have now obtained what they believe are the first clues to what might have caused the unprecedented deaths. Field observations had shown that female saigas, which cluster together to calve their young, died first, followed by their calves, which were still too young to eat any vegetation. 

Tissue samples obtained from the carcasses revealed that the toxins produced by Pasteurella and possibly the Clostridia bacteria had caused extensive internal bleeding in animals -- leading to their eventual deaths.

However, Pasteurella is found normally in the bodies of ruminants like saigas, and it does not cause harm unless the animals themselves have weak immune systems.

As herds separated by a distance of hundreds of miles suffered simultaneously, scientists now think environmental factors -- in the form of cold winters and wet springs -- might have played havoc with the animals’ immune systems.

“There is nothing so special about it [Pasteurella]. The question is why it developed so rapidly and spread to all the animals,” Zuther told LiveScience.