Nearly half of the world's population of a rare, central Asian antelope died in late May. And even the scientists and conservationists most interested in the animals are left scratching their heads as to why the deaths swept across central Kazakhstan so swiftly and with such ferocity.

Upwards of 120,000 of the critically endangered saiga antelope, which roam the grasslands of Russia and Kazakhstan and number about 250,000, have died since the first fatalities were reported on May 13. Four herds were hit by the mysterious aliment and every animal died in two of them.

"You just don't expect that," Richard Kock, a wildlife veterinarian with the Royal Veterinary College, told VICE News. "In a free-ranging population, there are major diseases and you get high mortality, but nothing like this."

Men load a trailer with carcasses of dead Saiga antelopes in a field, about 373 miles southwest from Uralsk town, in western Kazakhstan. (Photo by Uralskaya nedelya/Raul Uporov/Reuters)

The number of deaths is expected to climb as a response team continues to count the carcasses, said Aline Kühl-Stenzel, terrestrial species coordinator with the United Nations Environment Program.

On Friday, Kazakhstan's Ministry of Agriculture announced that the deaths appeared to have stopped.

"I'm not sure I want to call that good news, but we're relieved at least," Kühl-Stenzel told VICE News.

A group of scientists, including Kock, rushed to Kazakhstan two weeks ago to collect tissue samples and perform necropsies on the dead animals. Theories about the cause of the die off range from opportunistic infections to nutrient-rich gasses that when consumed by the animals caused a toxic reaction.

The saiga die off hit during the spring calving period, when females give birth, which can leave them weak and susceptible to infection, said EJ Milner-Gulland, chair of the Saiga Conservation Alliance.

"There's all sorts of factors that kind of come together, which is why we don't really know what the answer is," Milner-Gulland told VICE News. "It seems like it's just a perfect storm."

It will likely be weeks before the response team in Kazakhstan completes their toxicology and virology studies.

Kock doubts a single disease is responsible for the saiga deaths. The animals died too quickly to transmit a pathogen through the whole herd, and the deaths affected herds that were miles apart.

They all showed similar symptoms, including respiratory ailments, diarrhea, and massive internal bleeding, particularly after giving birth, Milner-Gulland said. Some were found foaming at the mouth.

"It's horrible, and very quick," Milner-Gulland told VICE News.

Major culling of the saiga population has occurred every decade since at least the middle of the 20th century.

But the current epidemic comes as conservationists struggle to revive the species after its population plummeted from more than one million a century ago to less than 50,000 in the early 2000s, largely due to hunting and habitat loss, according to the World Wildlife Fund.

Ninety percent of the saiga population lives in Kazakhstan, according to the Ministry of Agriculture. And with the deaths overwhelmingly affecting females and their calves, and populations outside of Kazakhstan struggling to survive, a saiga rebound is now in serious doubt.

"The tragedy of course is this is a recovering population. They're critically endangered," Kock told VICE News. "These sort of losses — I'm not sure they can take too many of them."

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