About 2,500 seals have been found dead on the Caspian Sea coast in southern Russia, officials say.
Officials say there is no sign that they died violent deaths.
Some 700 dead seals were initially reported, but further investigation has revealed a much higher number and counting continues.
Caspian seals have been classified as endangered on the International Union for Conservation of Nature (IUCN) red list since 2008.
Zaur Gapizov, head of the Caspian Environmental Protection Center, said in a statement that the seals probably died about two weeks ago.
There is no evidence to suggest the animals were killed or caught in fishing nets, he said.
The Dagestan region’s ministry of natural resources and environment wrote on Telegram that a large number had been found between the mouths of two rivers, the Sulak and the Shurinka.
Experts have collected samples from the seals and the ministry said reasons for the deaths will be established when lab results come back.
The seal population of the Caspian Sea has dwindled drastically over the decades from over-hunting and industrial pollution.
The Caspian Environmental Protection Center puts the number now at just 70,000, down from more than one million around a century ago.