Knut, the polar bear who became world famous as a cub, was found dead in his enclosure at the Berlin Zoo Saturday afternoon.
The death of the bear, who was only four years old, shocked zoo employees, as the animals can live to be about 30.
Knut was found floating in the pool inside his enclosure he shared with three other bears, including his mother, and the cause of death is not known.
Knut and his brother were rejected by their mother. His sibling died but Knut was raised by keeper Thomas Dörflein, who bottled-fed him, slept with him and played guitar for him. Dörflein died from a heart attack in 2008, at the age of 44.
In October, 2010 a Digital Journal story reported that Knut was not being treated well by the three female bears who shared the enclosure with him.
Knut was extremely popular as a cub. He was filmed, photographed, and toys were made to resemble him.
The BBC reported that, in 2007, Knut generated more than 5m (£4.4m; $7m) euros in extra income for Berlin zoo, through increased ticket sales and Knut-branded merchandise.
Dozens of people saw the bear fall and die.
One witness told BZ that Knut turned around in circles several times, then acted as if he were having a seizure and fell backward into the water.
Witnesses said he surfaced and reached his front paws above his head before dying. People called for help and one elderly woman fainted.
At the time Knut was spending his daily time alone in the enclosure.
"Knut's short and distressful life shows us again that polar bears do not belong in zoos, even if they are called Knut," The Local quoted Wolfgang Apel, head of the German animal protection association, as saying.
BZ reported that, following an autopsy, Knut's body is to be stuffed and placed in a museum.