Makhachkala, Russia - Russian authorities initially reported that some 700 seals had died and washed up on the shores of the Caspian Sea, but the actual number of animals is closer to 2,500.

On Sunday, Russian authorities said that 2,500 animal carcasses had been discovered at various locations in and near Makhachkala, the coastal capital of Dagestan in the North Caucasus.

When the authorities first shared videos and photos of the mass mortality incident on Saturday, they said that some 700 Caspian seals had died.

In initial reports, they noted that cause of death was unclear and the investigation is ongoing. They also warned that the number of deaths could rise.

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"This is the largest mass mortality of the Caspian seal in the past 10 years. The reasons will be determined after conducting laboratory tests of the pathological material of the dead animals," the authorities said, according to the Russian state news agency TASS.

The death of 2,500 seals is a significant loss for the struggling species.

The Caspian seal is an endangered species and is under special protection. It's been on the International Union for Conservation of Nature (IUCN) red list since 2008 and, per CNN, is the only mammal found in the Caspian Sea.

The Caspian seal population has declined by about 90% in the past 100 years, according to animal rights activists. Due to oil extraction, the Caspian Sea has also become more and more polluted. According to experts, the animals are dying from oil leaks, poaching, and overfishing.