Six whales believed to be part of a pod that was rescued from a mass stranding in southwestern Australia earlier this week have re-beached themselves and two have died, the government said Wednesday.

The four other long-finned pilot whales appeared to be deteriorating rapidly, the Western Australia state conservation department said.

Veterinarians were being sent to euthanize the ailing animals, which were spotted by airplane on a beach about four miles away from where a pod of 10 had been released a day earlier.

"We believe they are part of the rescued group from yesterday, so it's very disappointing," said John Carter, operations officer with the department. "We'll compare photos and measurements when we can get out there."

He said they thought the other four rescued whales were safe at sea.

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"At this stage we're assuming they are OK but we'll be monitoring the coast and waters over the next few days," Carter told The Associated Press.

The whales were part of a group of about 90 whales and five bottlenose dolphins that became stranded on a beach in Western Australia state early Monday. Most of the animals died, but rescuers were able to push four dolphins and four whales out to sea at the stranding site and truck 10 surviving whales overland to deeper waters Tuesday.

The saved whales appeared disoriented at first, trying to swim back to shore, but rescuers guided them to deeper waters and the animals began swimming away. Officials had hoped they were swimming to safety.

Carter said three dead whales were reported Wednesday on the beach near the site of the original stranding. But they were part of the original group, not saved whales returning to shore, he said.

At least one carcass along the beach had been chewed by sharks, and the beach would remained closed to swimmers until the danger of attacks passed, the department said.

This week's mass beaching was the fifth in Australia in as many months; nearly 500 whales have died.

Scientists say the types of whales that beach themselves are extremely social groups that follow pod members into danger. But they cannot explain what draws the deep-sea animals so close to shore.

There are a number of possible theories. The whales may be chased by predators such as killer whales, or they could be following prey themselves. The sonar they use to navigate the dark seas could be hindered by natural geomagnetic factors such as iron ore deposits. They may swim into an area where sandbars or peninsulas block their exit. Or they may follow one ill or injured pod member.

Human activity such as undersea exploration for petroleum or the sonar of submarines can also interfere with whale and dolphin navigation.

Whatever the reason, once one animal heads for the dangerous shallows, the rest are likely to follow.

"Certain species of whales are more prone to mass strandings because the social bonds between them are incredibly strong," said Mike Bossley of the Whale and Dolphin Conservation Society. "If one animal is in trouble, the others won't leave him."

Once stranded, some are battered by rocks and surf, while others die of overheating or have their organs crushed by their own body weight after leaving the buoyancy of water.

The mass strandings occur most often in the island state of Tasmania, in Australia's southeast, and in Western Australia.