Bear keeper Heiner Kloes from the zoo said the four-year-old was alone in his compound. He says the cause is not yet clear
Knut, the world-famous polar bear, collapsed and died in front of 600 visitors at Berlin Zoo this afternoon.
The bear rose to stardom when he was hand-raised by zoo keepers after being rejected by his mother at birth.
Bear keeper Heiner Kloes from the zoo said the four-year-old was alone in his compound. He says the cause is not yet clear.
Knut with the keeper Thomas Doerflein who cared for him when the bear's mother rejected him. Mr Doerflein died of a heart attack three years ago
'He was by himself in his compound, he was in the water, and then he was dead,' said Mr Kloes. ' He was not sick, we don't know why he died.'
A post mortem will be conducted on Monday to try pinpoint his cause of death, he said.
Berlin Mayor Klaus Wowereit called Knut's death 'awful'.
'We all held him so dearly,' he told daily newspaper B.Z. 'He was the star of the Berlin zoo.'
Birthday boy: By the time of his first birthday in December 2007, Knut had already become an international celebrity
The polar bear rose to global fame after he was rejected by his mother when he was born in captivity on December 5, 2006. The fluffy cub was shown to the public 15 weeks later, and attendance at the zoo has roughly doubled since, officials said.
The resulting 'Knutmania' led to a 2007 Vanity Fair cover with actor Leonardo DiCaprio shot by famed photographer Annie Leibovitz.
Though the zoo has never released exact numbers, Knut merchandise including postcards, key chains, candy and stuffed Knuts have brought in hundreds of thousands of euros.
Popular: Knut plays with a branch at the Berlin Zoo last year. He attracted thousands of visitors
Although Knut was the first polar bear to be born and survive at Berlin Zoo in more than 30 years, his tragic young life was blighted by tragedy from the start.
Both he and his twin were rejected by their mother at birth and his unnamed brother died four days later.
But Knut amazed medical staff at the zoo by clinging to life. After spending his first 44 days in an incubator, he was put in the care of zoo keeper Thomas Dörflein, who began raising the infant with round-the clock care.
Knut's fame grew to such an extent that he was featured on a set of stamps issued by the German Post Office
News of the cub's miraculous survival was picked up by the world's media and the young cub became the zoo's top attraction.
But tragedy struck again before Knut's second birthday in 2008, when Mr Dorflein died of a sudden heart attack at the age of 44.
Observers noted that the bear appeared to suffer from depression in the weeks after his beloved keeper and constant companion died.
Experts had earlier warned that the Knut was developing into a 'psychopath' who was too dependent on human contact.
In 2009 Knut was the subject of a bitter custody battle between two zoos. Neumuenster Zoo had loaned Berlin Zoo Knut’s father Lars in 1999 on the condition that the first-born cub from his union with mother Tosca would belong to Neumuenster.
And last year Knut's cousin died after a mystery virus spread through the zoo's bear enclosure.
source: dailymail