The United Nations Environment Program (UNEP) confirmed reports Thursday of a widespread die-off in critically endangered saiga antelope throughout central Kazakhstan, reporting that more than 120,000 of the animals have died since mid-May.

This represents more than a third of the global population — and half of a local population — making this the largest die-off event of the species ever recorded.

In a press release, the U.N. called the deaths "a major blow for conservation efforts," since saiga antelopes had been recovering, up from less than 50,000 animals globally during the past decade. Back in the 1970s, this antelope species numbered above one million, according to the IUCN Red List of Threatened Species, but this number declined due to habitat loss and poaching.

While scientists have not yet pinpointed the cause of the deaths, a preliminary analysis shows that a combination of environmental and biological factors is involved, with mainly mothers and calves among the dead.

Four large birth groups of saigas were wiped out, the U.N. says, noting that "not a single animal survived in the affected herds."

“This loss is a huge blow for saiga conservation in Kazakhstan and in the world, given that 90% of the global saiga population is found in our country. It is very painful to witness this mass mortality," said Erlan Nysynbaev, the vice minister of agriculture in Kazakhstan, in a statement.

"We established a working group that includes all relevant experts, including international ones, and are determined to identify the causes and undertake all possible efforts to avoid such events in the future.”

Kazakhstan has brought in the secretariat of the Convention on the Conservation of Migratory Species of Wild Animals (CMS), which despatched an emergency mission last week with experts from the Royal Veterinary College in the United Kingdom and the U.N. Food and Agriculture Organization to conduct post-mortem analysis and try to solve the mystery of why these animals were dying in such large numbers.

The Convention on the Conservation of Migratory Species of Wild Animals is a U.N. treaty that aims to conserve migratory species worldwide, and to-date, 120 nations have ratified it.

According to members of the CMS mission, two pathogens, one known as Pasteurella and the other as Clostridia, are contributing to the rapid and wide-spread die-off. However, this discovery fails to solve the mystery, since these bacteria typically only kill animals with previously-weakened immune systems.

“Experts are working around the clock to investigate the impacts in terms of wildlife health of the relatively high rainfall observed this spring, the composition of the vegetation and other potential trigger factors including a suite of viruses," says Aline Kühl-Stenzel, who is a land species expert with the UNEP.

Dead Saiga antelopes in Kazakhstan. Image: Sergei Khomenko/FAO

One possible suspect in the die-off event is rocket fuel from decades of launches from facilities in central Kazakhstan. Currently, Russia launches crewed and uncrewed missions to the International Space Station from the Baikonur Cosmodrome in Kazakhstan.

Kühl-Stenzel says, however, that the data gathered so far does not point to rocket fuel as a cause of the die off.

Before this mass annihilation, the population of saigas in this area, known as the Betpak-dala population, stood at about 250,000 animals. This means that the die-off event halved the population.

Final estimates may exceed 120,000 dead saigas, the U.N. warned, since workers are still counting the dead animals. Fortunately, though, they think the event is over, at least for now.

While mass mortality events are not unusual for saiga antelopes, they typically affect far smaller numbers of animals, on the order of about 10,000 saigas. The magnitude of this event, therefore, is unprecedented, given the population's large size.

"Often these mass mortality events occur in the birth period, when saiga females come together in vast herds to all give birth within a peak period of less than one week," the U.N. press release stated.

There is hope that this species could bounce back relatively quickly, since saiga antelopes often have twins.