(EASTHAM, Mass.) — Five pilot whales that were stranded on a Massachusetts beach have been returned to the ocean, but they may still be in peril, according to experts.

Conservationists from the International Fund for Animal Welfare’s Marine Mammal Rescue and Research team were sent to Sunken Meadow Beach in Eastham, Massachusetts, Monday night after six live pilot whales were spotted swimming close to shore around 4:45 p.m. ET, just after dark, Stacey Hedman, communications director for the research center, told ABC News.

All of the whales were briefly examined, and two were given satellite tags, Hedman said.

By Tuesday morning, the whales had become stranded on the beach, and one — a calf — had died, Hedman said.

Some of the animals are “very large,” with the largest estimated to weigh about 4,000 pounds, Brian Sharp, director of the research center, said in a recorded statement.

Teams of rescuers were responding in phases Tuesday to provide supportive care until the tides were more favorable, Hedman said.

The conservationists hoped that higher tides after 3:30 p.m. would help push the whales back out to sea, and all five were re-floated and released shortly after, according to the IFAW.

However, four of the whales turned back to shore, and by 6 p.m., rescue efforts were put temporarily on hold, the organization said.

“The five pilot whales swam off well in one direction together, but the reality is that we cannot celebrate a success yet this evening,” Misty Niemeyer, stranding coordinator at IFAW, said in a statement. “One animal is now offshore, but the others did not follow.”

The team will evaluate the next steps on Wednesday, Niemeyer said, describing the rescuers as “exhausted” after their strenuous efforts on Tuesday.

“Large animals can be quite dangerous to work around, and it’s for our health as well as tomorrow’s continued efforts that we need to call it a day today,” she said.

Video taken on the scene showed crews digging up sand around the whales, some of which were covered in wet blankets to help them retain moisture. Some of the whales were also administered fluids via IV to help combat the stress and shock of stranding, Sharp said.

Dolphins and small whales can indeed live out of water for many hours when receiving proper supportive care and hydration, Hedman said.

While Cape Cod is considered a global hotspot for live cetacean stranding, historically, pilot whales do not strand there, Hedman added.

IFAW typically transports dolphins to deeper water using a custom-built rescue vehicle, but the whales are too big to transport, according to the organization.

“[We’re] doing everything we can right now to give these animals the best shot at the best outcome,” Sharp said.

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