Knut, the celebrity polar bear who died at the weekend, may have succumbed to a brain disorder, Berlin zoo said on Tuesday.
Knut, who won global fame as a cute cub but grew into a 200 kg predator, passed away in front of horrified visitors to the zoo on Saturday. He was just four.
With fans demanding to know the reason for his sudden death in his pool, Berlin zoo ordered an autopsy from the Leibniz Institute for Zoo and Wild Animal Research.
"The initial results show significant abnormalities in the brain which could be regarded as the reason for the polar bear's sudden death," the zoo said in a statement.
Knut was rejected by his mother and hand-reared instead by his keeper Thomas Doerflein. Thousands of visitors came to watch keeper and cub playing together, and Knut''s fame spread around the world. Doerflein also died young, in 2008.
Other tests on Knut will last several days but the zoo promised to release the final autopsy results immediately.
Once a white fluffy ball of energy, the German post office produced a stamp in his honour and Knut souvenirs became a huge business. By the time Knut had reached adolescence, he had generated 5 million euros ($7 million) for Berlin zoo.
A custody battle broke out over Knut, with Neumuenster zoo staking a claim - or at least a slice of the profits - as it had lent Knut's father, Lars, to Berlin.
Knut's fan club shrank as the cuddly orphan grew up and last year an animal rights group said he should be castrated because he was related to his love interest, Giovanna.
Animal rights campaigner Frank Albrecht, who made the suggestion, said he had received many angry calls, including one from a man saying he should castrate himself before the bear.
