Police Captain John Greland said on Twitter that most of the animal was “below surface.” It was unclear at first, he said, if the animal was a shark or whale.

Officer Rachel McGuire, a police spokeswoman, said the department’s harbor unit received a call about the carcass around 8 a.m. The deceased animal was near the harbor unit’s headquarters.

Boston police and the Coast Guard Tuesday checked out the carcass of a 24-foot-long basking shark that was found floating just beyond the Black Falcon Cruise Terminal in South Boston.

Greland posted pictures and a short video of the animal as officials tried to bring it toward a boat.


Two Coast Guard units were also sent to the scene to help investigate the shark’s remains.

Tony LaCasse, a spokesman with the New England Aquarium, which responds to deceased and beached marine wildlife, said the carcass was 50 yards offshore.

He said the animal was first sighted on the bow of a container ship entering Boston Harbor two weeks ago last Friday. The carcass came free from the vessel, and likely sank before reappearing Tuesday.

“We were expecting it to pop up sooner,” he said. “The important thing to remember is that it’s possible the animal could have been dead before the vessel had struck it” and then dragged it toward the inner harbor.

LaCasse said experts determined the carcass was a basking shark. The second largest shark in the world, basking sharks frequent New England waters this time of year.

LaCasse estimated that at the time it was alive, the shark was likely around 24 feet long and weighed 3 to 5 tons.

Officials looked at the shark’s pectoral fins and other cartilaginous features, like its gill rakers, to identify the species. It was unclear how the animal died.


“[The gill rakers] are a mechanical food filter for the sharks,” who don’t pose a threat to humans, and feed on plankton, LaCasse said.

LaCasse said the animal’s remains were left in the water to float back out into the harbor.

Earlier this month, a North Atlantic right whale calf was found dead off Morris Island in Chatham.

On April 26, the carcass of a young humpback whale washed ashore in Duxbury. The female whale was underweight and had an injury around her mouth.

Steve Annear can be reached at steve.annear@globe.com. Follow him on Twitter @steveannear.