Initial findings from a necropsy performed Monday by an institute in the German capital showed "significant changes to the brain, which can be viewed as a reason for the polar bear's sudden death," the zoo said in a statement.
The zoo didn't elaborate on the changes to the animal's brain, and officials could not immediately be reached for further comment.
Pathologists found no changes to any other organs, the zoo said, adding that it will take several days to produce a final result. Further planned tests include bacteriological and histological, or tissue, examinations.
Knut died Saturday afternoon in front of visitors at the zoo, turning around several times and then falling into the water in his enclosure. Polar bears usually live 15 to 20 years in the wild and even longer in captivity.
Knut, who was born in December 2006 at the Berlin zoo, rose to celebrity status as an irresistibly cute, fluffy cub.