A critically endangered species of antelope has been driven further to the brink of extinction following a mysterious and substantial die-off of its herd over the summer. The herd of saigas, which are native to regions of Kazakhstan, Russia and Mongolia, began dying during their calving period in May, reports Newser, and scientists are at a loss to figure out why. According to reports, some 12,000 saigas died from a still-unexplained digestive issue after scientists speculated that there might have been too much greenery, reports Live Science.

However, the death rate quickly accelerated to 120,000 animals, or about half of the population in Kazakhstan, by June, including a herd of 60,000 that died off in about four days. “I have worked in veterinary diseases all my career and I have never seen 100% mortality,” Richard Kock, a wildlife veterinarian at the Royal Veterinary College in Hatfield, UK, who flew out to Kazakhstan last month to assist with efforts to make sense of the devastation, told Nature. “We had a herd of 60,000 aggregated and they all died. That is extraordinary.” FULL REPORT