Amateur video of Knut's death surfaces

Video has hit the Internet of the death of the beloved, famed polar bear, Knut, at the Berlin Zoo. Knut was in the enclosure Saturday afternoon when he suddenly collapsed in front of more than 600 visitors. He was only 4 years old.

The amateur video shows the bear walking in circles, shaking and then collapsing into the water. The bear thrashes in the water for a few seconds and then, the water suddenly goes still.

Click the video below to watch the amateur footage:

Knut, world-famous polar bear, found dead

Zookeeper Heiner Kloes told the press, zoo staff was shocked. One moment, he was in the water, and the next, he was dead. He said Knut wasn't sick, and they don't know why he died.

Pictures: Nuts over Knut
Pictures: Knut's first milestone

On "The Early Show" Monday, CBS News Correspondent and resident veterinarian Dr. Debbye Turner Bell reported Knut captured the hearts of millions around the world in 2007. As a cub, Bell said, Knut's mother rejected him, leaving him in the hands of zoo keepers. The only polar bear to ever be raised by humans, Knut became an instant sensation with three million visitors a year, his own feature film and even a cover shot on Vanity Fair magazine.

Bell reported the adorable cub grew attached to his human caretakers. However, as he got older -- and bigger -- he became too large and dangerous to interact with humans, prompting critics to speculate he was lonely and depressed.

"The Early Show" visited Knut in 2008, and asked his keepers if he was living in a healthy environment.

Berlin Zoo vet Andre Schuele said at the time, "He's not addicted to humans and he's not missing the keepers and he's happy with this situation."

As visitors mourn the loss of the adorable bear and veterinarians prepare for a necropsy -- the animal version of an autopsy -- animal rights activists are placing blame on the zoo.

Ashley Byrne, a senior campaigner for People for the Ethical Treatment of Animals (PETA), said, "Knut was observed displaying behaviors that indicated he had gone mad from confinement. He paced constantly and swayed, and it was obvious to anyone who knew what to look for that captivity was driving him insane."

Bell remarked, considering polar bears typically live up to 20 or 30 years in captivity, no matter what the reason behind Knut's death, one thing is clear: It came too soon.

Bell said today's necropsy on Knut may reveal some preliminary findings if the cause of death is a structural defect to his anatomy, such as a heart abnormality. But she said it could be days or weeks before a final determination can be made.

She added, "For a young, hardy bear, all indications was that he was healthy, his behavior was normal and he had reached his teenage years, his prime. So he should not have been a candidate to die. When they're very young, they're vulnerable to death. But he'd gotten past that point."

Co-anchor Chris Wragge said, "The animal rights activists who feel maybe the treatment there, living in captivity is not good for these animals. You were out there in Berlin, what did you think?"

Bell said, "I absolutely was. I saw him. He seemed to be a normal, healthy young bear. He didn't (display) the behavior that they are talking about. In fact, he mugged for the visitors whenever there were people there. He came up. He waved. He stood up. He swam in the pool to get as close to them as possible. Very curious. So, it's unlikely that his circumstances, being in captivity, is the reason for his death. Of course, we won't know until they actually do the postmortem and get the results back."

Bell explained a necropsy is very similar to an autopsy.

She said, "They're going to take samples of tissue. They're going to take blood samples. They'll test for toxicities, look for structural defects. And hopefully, we'll know something."