The adult whale's carcass was found on a Nantucket Island beach and left there while preparations were made to conduct a necropsy that had been set for Monday, The Boston Globe reported.
However, a check on the whale Sunday night by Nantucket police found someone had cut through the whale's tail, almost completely removing it.
Dead whales are usually moved and transported by their tails. Because the tail was so badly damaged authorities were unable to move the 30-ton whale before unusually high surf carried it back into the ocean, Tony LaCasse of the New England Aquarium said.
Parts of whale carcasses often disappear, LaCasse said. Sperm whales are often found with their jaws missing as their teeth are highly prized by some, he said.
An attempt to remove the tail of a humpback whale is far less common, he said.
"For someone to take a tail from a humpback whale, that's an unusual one,'" LaCasse said. "We're a little puzzled."
Mutilating a whale, dead or alive, is a serious federal crime with stiff fines for violating the Marine Mammal Protection Act and the Endangered Species Act, wildlife officials said.
"It would be very challenging to find the person who did it,'" said Kate Sampson, sea turtle stranding coordinator at the National Oceanic and Atmospheric Administration.
"There's really no evidence left at the scene of the crime."