They performed a necropsy Monday and results are expected by Tuesday. Knut died Saturday afternoon in front of visitors, turning around several times and then dropping to the ground, and falling into the water in his enclosure at the Berlin zoo. He was just four years old.
Polar bears usually live 15 to 20 years in the wild and even longer in captivity.
Knut was rejected by his mother at birth, along with his twin brother, who only survived a couple of days. The bear went on to appear on magazine covers, in a film and on mountains of merchandise.
Fans are now paying tribute to Knut by placing flowers and pictures outside his enclosure, as well as signing their name in a condolence book in tribute to Knut.
"He has brought joy to us, the Berliners and many others around the world," the zoo said in a statement. "Knut also was an icon for the endangerment of his species and natural habitats of all wild animals."
In addition, the zoo said it was starting a special account to accept donations on Knut's behalf, which will be used for polar bear research and the preservation of their habitat.
The Associated Press contributed to this story