A marine mammal expert conducted a necropsy Sunday on the whale that wandered into the River Thames, hoping to determine what caused the 20-foot-long animal to veer off course and splash through central London before dying during Saturday's rescue attempt.

The Zoological Society of London said it hoped preliminary results on what killed the Northern bottlenose whale would be available Wednesday. Paul Jepson, who has conducted government-funded research into why dolphins and whales strand themselves on British shores, was performing the examination, the society said.

The whale captivated onlookers as it swam in the shallow, murky waters of the River Thames past the Houses of Parliament and Big Ben. It died Saturday night after rescuers tried to carry it into deeper waters of the North Sea, swaddled in blankets on a rusting salvage barge.

Thousands of onlookers had lined the banks of the river and jostled for space on bridges to watch the whale being lifted by crane into the barge. The drama was broadcast live around the world.

Earlier, the whale twice tried to beach itself. Experts said the whale died Saturday after suffering convulsions and struggling with the effects of being out of the water.

It was the first sighting of a Northern bottlenose in the Thames since records began in 1913.

The Zoological Society said Jepson would look for signs of damage to the whale's skin before sending blubber samples for analysis. He would then examine the whale's internal organs and the echo response areas of the brain, which may reveal why the mammal became lost.

The whale was about 40 miles from the mouth of the Thames on the North Sea.

Tony Woodley, a director of the British Divers Marine Life Rescue group, defended the attempt to move the whale to deeper waters.

"We believe that if the whale would have been left how it was then it would have just slowly died and we don't think that was the acceptable option to take," said Woodley, whose group led the rescue effort. "We always knew that it was going to be risky. We did everything that we could and I am afraid that this time it was not a success."

The Northern bottlenose whale can reach nearly 30 feet in length — longer than a traditional red double-decker London bus — and weigh nearly 8 tons.

The whales are known as curious animals, readily approaching boats and normally traveling in groups, according to the Whale and Dolphin Conservation Society's Web site. When sick, old or injured, whales often get disoriented and swim away from their pod.

The sight of the whale swimming past London's famous landmarks bemused thousands of onlookers in the British capital. Witnesses reported seeing a second whale in a different section of the Thames on Friday.

Scientists have said fluctuating ocean temperatures, predators, lack of food and even sonar from ships can send whales astray into potentially dangerous waters.

Woodley said it was too early to say what caused the whale to become lost, and he dismissed as speculation suggestions the mammal may have been disoriented because of sonar signals from navy ships in the North Sea.

"It is generally accepted that the animal was lost, being away from its normal environment of the deep sea Atlantic," he said. "But until the post-mortem is completed we can't tell if it had major internal problems or not."