Alaska - Nine endangered fin whales have been found dead in Alaskan waters in recent weeks, and researchers said on Friday they were searching for what might have killed them.

The dead whales were discovered in waters from Kodiak Island, off the south coast of mainland Alaska, to Unimak Pass at the eastern end of the Aleutian Islands, researchers from the University of Alaska Fairbanks said in a statement.

The number of dead whales reported since late May has vexed researchers who typically see no more than one fin whale carcass every couple of years, said Kate Wynne, an Alaska Sea Grant marine mammal specialist with the university. ‘It is really perplexing for a number of reasons,’ Wynne said.

‘They appear to have all died around the same time. And the strange thing is they are all one species, with the exception of one dead humpback whale found in a different location.’

After the blue whale, the fin whale is the second largest mammal in the world. The whales, found in deep waters of all major oceans, have been listed as endangered in the United States since 1973. Scientists from the Alaska Sea Grant and National Oceanic and Atmospheric Administration were looking for more carcasses, and were testing water samples for temperature and any evidence of a possible dangerous algae bloom, the statement said.

The whales had no signs of injuries and showed healthy layers of blubber. ‘So far there is no ‘smoking gun’ in this environmental mystery,’ Wynne said. Fin whales live in social groups of two to seven and live an average of about 80 years, according to NOAA Fisheries. Adults can grow up to about 70 feet (21 meters) long and weigh around 45 tons.