A dead whale measuring more than 50 feet long was found floating in Boston Harbor, and wildlife experts were trying to determine what killed the massive mammal.

The finback whale was found early Sunday morning, according to the Boston Globe, which cited experts who called the find rare. Tony LaCasse, a spokesman for the New England Aquarium, said biologists were trying to find the cause of death.

“We don’t know,” LaCasse told the paper. “We’ve only been able to look at a small portion of the body so far.”

Most likely, the whale died of natural causes, disease, or some man-made trauma such as being struck by a boat or getting tangled in fishing nets. Blood in the water around the beast could indicate trauma, he said.

Adult finback whales can be up to 70 feet long and weigh as much as 45 tons. Authorities plan to conduct tests on samples taken from the animal, and to perform a necropsy. They will wait for the whale to wash ashore.

“It’s not rare to see a whale in the Boston Harbor, but it’s rare to see a dead whale in the Boston Harbor,” said Brian Fleming, command duty officer at the US Coast Guard Base Boston. “The majority of these cases happen out to sea.”

LaCasse told the Globe the whale floated to within “a couple hundred yards” of the shore of Long Island in Boston Harbor at about 3:30 p.m. Sunday, but the tide carried it back into the harbor.

Click for more at The Boston Globe.