U.S. government biologists have launched a special investigation into the deaths of at least 70 gray whales washed ashore in recent months along the U.S. West Coast, from California to Alaska, many of them emaciated, officials said on Friday.

The National Oceanic and Atmospheric Administration (NOAA) declared the whale die-off an "unusual mortality event," a designation that triggers greater scrutiny and allocation of more resources to determine the cause.

So far this year, 37 dead gray whales have turned up in California waters, three in Oregon, 25 in Washington state and five in Alaska, say officials of NOAA's National Marine Fisheries Service. Five more were found in British Columbia.