Herbert Knosowski / AP file
Knut, the polar bear cub, has its first public appearance with his keeper Thomas Doerflein in the Berlin zoo on March 23, 2007. A Berlin zoo official says world-famous polar bear Knut has died. Bear keeper Heiner Kloes said that four-year-old Knut died Saturday, March 19, 2011 while alone in his compound. He says the cause is not yet clear.