They determined the cause of death to be some brain disease, or disease that affected the brain. Some closer examinations are expected to determine the cause more clearly and should be published within the next days.
Last December, coincidentally the same month Knut was born, two polar bear cubs were born at Nuremberg Zoo who have now made their first public appearance (also by accident just days after his passing).
Picture This: Reincarnation? - SPIEGEL ONLINE - News - International