(CNN) -- Officials may never know what killed a whale found floating in Boston Harbor as tides began to carry it toward the sea on Monday.
The whale, a member of the endangered Finback family, measures more than 50-feet long. It was discovered on Sunday and was seen in harbor waters close to downtown at one point.
But it has since floated away from shore and there was a strong chance the tides would carry it to the Atlantic Ocean, said New England Aquarium spokesman Tony LaCasse.
LaCasse said the cause of death was unknown and the plan was to let the whale drift out to sea.
While whale sightings in Boston Harbor have occurred in the past, dead whales in those waters are uncommon, Coast Guard Petty Officer Robert Simpson said.
"That usually happens out at sea," he said.
Connie Barclay, spokeswoman for the National Marine Fisheries Service, said whales can get tangled in fishing lines or be struck by ships, in addition to dying from natural causes.
For officials to determine a cause of death in this case, the whale first would have to wash ashore. That would enable experts to conduct a necropsy, or an autopsy for animals.
The Finback is the second-largest whale in the sea and is on the endangered species list because of Japanese whaling, LaCasse told CNN.
A dead Finback washed up on a New Jersey beach earlier this year. Officials suspected that whale was hit by a ship.