The only polar bear to ever be raised entirely by humans and to have his own Vanity Fair cover photo shoot died suddenly on Saturday, March 19 in front of hundreds of his many fans at the Berlin Zoological Garden.

Knut born on December 5, 2006 was only four years old when he collapsed, leaving grieving admirers all over the world to cope with his early death.

The zoo ordered a necropsy to determine why the bear died so young.The autopsy revealed he died from brain damage which had not been detected. Most polar bears live 20 to 30 years in captivity.

The Associated Press reports that mourners flocked to the zoo on Sunday "laying down red roses and white stuffed polar bears, lighting candles or putting up pictures of Knut with personal messages for him." Children wrote farewell poems for him, and a group of "die-hard" fans stayed all day, many of them crying. "I've been crying nonstop since I heard about his death," Ingrid Rommel told the AP.

She said visiting Knut weekly since 2006 helped her get over her husband's death.

Thousands of people signed the condolence book opened by zoo authorities for Knut.

Animal rights activists are raising questions about Knut's treatment at the zoo, saying zookeepers turned a blind eye to the cub's problems because he was such a popular draw for the facility.

In 2007, the Berlin Zoo's profits spiked 27 per cent from Knut, who brought in millions of dollars in revenue.

A spokeswoman for People for the Ethical Treatment of Animals (PETA) told the Daily Mail Knut was being bullied by three female bears 'Tosca, Nancy and Katjuschaand should have had his own space away from them."

PETA Germany repeatedly asked zoo authorities to move Knut away from the three females to a different location. His premature death could possibly have been avoided," PETA said.

Peter Ewins, an arctic species specialist with the World Wildlife Fund, told ABC News some animals react badly to captivity. "Just the effects of being in captivity, it's more of neurological, psychological thing that affects some animals," he said.

Knut was raised by zookeeper Thomas Doerflein after the cub's mother rejected him. Doerflein died from a heart attack in 2008, around the same time that one of Knut's keepers warned he was becoming a "psycho" bear.

Doerflein had been banned from playing with Knut because he had become an adult bear and it was too dangerous, leaving Knut sad and lonely."Knut needs to leave Berlin Zoo, the sooner the better. He's had so much contact with humans in his young life that he doesn't realise he's a polar bear, he needs to get used to a life without the people who brought him up," keeper Markus Roebke told German tabloid Bild Zeitung in 2008, according to ABC.

Roebke said the bear was addicted to fame.

"He actually cries out or whimpers if he sees that there is no audience outside his compound ready to 'ooh' and 'aah' at him," he said.Meanwhile, the famous bear's fans on Twitter and Facebook are asking why the bear died, and celebrating his short life.

"You were far too young to go. You touched everybody when you were the sweetest baby. Now you are missed by so many people! RIP, sweet, big boy!" wrote one mourner.