Mystery Surrounds 'Unusual' Death of 30 Large Whales in Alaska

— -- It's a whale of a mystery.

Thirty large whales have died in the western Gulf of Alaska since May, now deemed an "unusual mortality event" by the National Oceanic and Atmospheric Administration (NOAA). The historical average for this time-frame is eight deaths, said Julie Speegle, a spokeswoman for NOAA in Alaska.

The animals included humpback whales, fin whales and a gray whale, NOAA said. They were found around the islands of the western Gulf of Alaska and the southern shoreline of the Alaska Peninsula.

The cause of the whales' deaths is unknown and now under investigation, NOAA said.

"NOAA Fisheries scientists and partners are very concerned" but "do not yet know the cause," Dr. Teri Rowles, NOAA Fisheries' marine mammal health and stranding response coordinator, said in a statement released by NOAA Thursday.

Speegle told ABC News today the "leading hypothesis at this point is that this was somehow caused by harmful algal bloom that's spreading up the West Coast this year."

But NOAA is still "investigating a wide range of possibilities," Speegle said.

Such investigations can require months or years of data collection and analysis, according to NOAA.

NOAA, the Alaska Marine Mammal Stranding Network and many of the organizations' partners are involved in the investigation, Speegle said.