The Berlin zoo says a test performed on celebrity polar bear Knut indicate that "significant changes" in the animal's brain led to his sudden death last weekend.
The zoo said Tuesday that initial results from the necropsy examination performed by an institute in the German capital did not show changes to any other organs.
It didn't elaborate but said further tests, including bacteriological examinations, will take several days.
The 4-year-old polar bear 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, and the zoo is hoping the investigation will clarify what happened.