Since the beginning of May, thirty large whales have washed ashore along the Alaskan coastline in what the National Oceanic and Atmospheric Administration has declared an “unusual mortality event,” the Guardian reports.

Fourteen humpback whales, 11 fin whales, and one gray whale have been found stranded on islands in Alaska’s western gulf and along the peninsula’s southern shore. Four other unidentified cetacean carcasses have been found. (Bears have been seen scavenging the corpses.)

In a statement, NOAA’s Dr. Teri Rowles said that scientists from the federal agency are “very concerned” about the number of dead whales—almost three times the historical average—and have opened an official investigation into the deaths.

According to Wired, declaring an “unusual mortality event” enables the NOAA to begin a formal investigation, with state, federal, and tribal partners.

“While we do not yet know the cause of these strandings, our investigations will give us important information on the health of whales and the ecosystems where they live,” Rowles said.

“Our leading theory at this point is that the harmful algal bloom has contributed to the deaths,” Julie Speegle, a NOAA spokesperson, told the Guardian. “But we have no conclusive evidence. The bottom line is we don’t know what’s causing these deaths.”

Of the 61 UMEs declared since 1991, causes have been determined in 29.

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