Knut, a four-year-old polar bear, was made famous when his mother, a former circus bear, rejected him and he was raised by zookeepers.
The cause of his death is still unknown. Knut collapsed suddenly on Saturday and fell into a pool of water in front of 600–700 visitors, where zookeepers found him dead, according to the German newspaper Deutsche Welle.
Animal rights groups speculate Knut died from stress. The polar bear had recently been put in a pen with three female polar bears, including his mother, who were not friendly to the human-raised Knut, according to the Salt Lake Tribune.
Polar bears in captivity usually live to be 15 to 30 years old. Veterinarians are examining Knut’s body for the cause of death.