Siberian authorities are investigating the mysterious deaths of more than one hundred seals at Russia’s Lake Baikal, the world’s largest freshwater lake.

Roughly 132 seal carcasses were found at three locations near the lake, the Interfax news agency reported Tuesday. The earless species native to Lake Baikal is commonly referred to as the nerpa in Russian. The nerpa population is currently estimated at approximately 100,000.

Environmental authorities told Interfax on Tuesday they plan to collect water samples at locations where the nerpas were found to determine whether they were poisoned.

Alexei Kalinin, an attorney at the West Baikal Environmental Prosecutor’s Office, said the samples would be sent to a lab in Irkutsk to establish whether a virus was to blame. The analysis will determine if they fell victim to the same disease that killed thousands of nerpas in the late 1980s.

Meanwhile, local veterinary officials told Interfax on Monday that cardiac arrest was the preliminary cause of death.

Reports earlier this month indicated that a tourist boom, growing pollution and lack of sewage treatment has led to the disappearance of local fish species and a growth of putrid algae in Lake Baikal.