The death of over 60,000 saiga antelopes in a four day period has experts stumped.

The endangered saiga population was already at a low in May with only 125,000. A team of researchers from the Altyn Dala Conservative Initiative arrived in Kazakhstan, led by geoecologist Steffen Zuther. Set out to monitor the calving of the saigas, they instead were witness to a mass die-off of the species, according to Tech Times.

This was not the first incident that attracted attention. Similar cases in other parts of Kazakhstan were reported attracting attention from conservationists between May and June.

The latest report said that researchers found some clues as to why the rapid deaths were happening among the saigas. Zuther said it could be that bacteria affected the herds, but at this time, it is still unknown what really happened.

But Zuther added that there is no record of this extent of a die-off every happening within any other species.

In an attempt to find some trace of something that could have triggered the deaths, field workers have been taking samples from the environment such as rocks, soil, water the animals drank from and from the vegetation they ate in the past few months. They also gathered ticks and insects that are known to feed on the species.

Necropsies of these animals were done by scientists and they found that the female saigas were actually hit harder than the males. And after the female saigas died, so did her calves. Scientists could conclude so far that whatever was killing the saigas, could be spread through the mother’s milk.

The tissue samples that were collected and analyzed found that Pasteurella and Clostridia could have produced toxins that led to internal bleeding in the saigas. But Pasteurella does not usually harm animals such as the saigas, unless their immune systems are already compromised.

Zuther thought that one of the possible causes could be just the cold, hard winter that has come after a wet spring where lots of vegetation and standing water on the ground might have enabled bacteria to spread quickly, as this scenario would not be unusual.

But still, another probability could be that of a response to some natural variations. Zuther and his team are continuing their investigation until they find a reason for what happened.