Watch Knut the celebrity polar bear's last minute alive as he dies in front of hundreds of visitors at the Berlin Zoo.Heiner Kloes, Knut's keeper at the Berlin Zoo, told the Associated Press, "He was by himself in his compound, he was in the water, and then he was dead. ...He was not sick, we don't know why he died."Scientists say it's impossible to know what led to Knut's death until after the postmortem, which zoo officials said will take place next week. They acknowledged that it is unusual for polar bears to die in captivity at such a young age."It's a little bit surprising," said Peter Ewins, an arctic species specialist with the World Wildlife Fund. "In captivity, polar bears can live longer than in the wild; to 25 or 30. Even more than 30 years old because they're not exposed to the elements and hard realities of life in the wild."For polar bears in the wild, the first real hurdle is birth, and some animals continue to struggle in their first year or two of life. But, at 4, Knut would have been like an 18-year-old human, Ewins said."He'd be coming into breeding age," he said. "He's supposed to have been in his prime."