A dead 40-foot whale washed up at South Beach in Edgartown on Friday. The sei whale may have been killed by a boat propeller.

David Grunden, the Oak Bluffs shellfish constable and member of the Island’s marine mammal stranding network, and his team investigated. Mr. Grunden believes the sei whale probably had been dead for a week or more.

Sei whales are a small version of the much larger finback whale. Adult sei whales measure as much as 50 feet in length. The whale is fairly common in spring and summer in Georges Bank and up around the Gulf of Maine.

While it is unknown what the cause of death was, Mr. Grunden said they found evidence the animal may have been hit by a spinning propeller behind its dorsal fin. “It may have been hit by a boat,” he said.

Mr. Grunden said the animal was rolling in the surf, east of the Left Fork parking lot.

Over the years a variety of different whales have washed up at that area of beach.

The New England Aquarium was notified.