Eastham, Mass. ? Forty-five pilot whales died Tuesday, most of them euthanized by exhausted and heartbroken marine experts unable to free the mammals from a salt marsh where they became stranded for a third time in two days.

Volunteers had worked feverishly to free the mammals, covering them with wet blankets and bedsheets to regulate their body temperature and herding them out to sea.

“After two days of trying to give these animals any opportunity we could, a decision was made by the veterinarians on site to euthanize those animals that weren’t already dead,” said New England Aquarium spokesman Tony LaCasse. “It’s probably one of the harder decisions that anyone can make.”

Since the strandings began Monday on Cape Cod, 56 whales have died or been euthanized.

Vacationers and volunteers first began working to push the whales out to deeper water Monday after they beached about 25 miles away at Chapin Beach in Dennis. The whales were tagged for identification.

But early Tuesday, 45 of the animals beached again at Lieutenant Island. About 300 people came to the area to try to help the whales. Volunteers put blankets and bedsheets on the mammals, while schoolchildren filled buckets of water to pour on the whales to keep them comfortable while awaiting high tide.

Fourteen of the whales died early Tuesday.

On Tuesday afternoon, the remaining 31 whales swam ashore again near Sunken Meadow Beach in a marshy, hard-to-access location.

This time there was nothing rescuers could do, LaCasse said.

Twenty-five whales were euthanized and the rest died.