Knut the polar bear dies in Berlin

The world's most famous polar bear Knut sits in his enclosure at the zoo in Berlin on December 10, 2010. Knut became a global sensation n when he survived rejection by his mother and attracted more than two million visitors to the zoo. The polar bear died unexpectedly Saturday, March 19, 2011.

Knut, the 3-year-old polar bear, rolls in the snow in his enclosure at Berlin's Zoologischer Garten zoo January 8, 2010.

The world's most famous polar bear Knut yawns in his enclosure at the zoo in Berlin on December 10, 2010.

Knut the polar bear stands up to greet the crowd on his first birthday and Berlin's Zoologischer Garten zoo December 5, 2007. The bear, born December 5, 2006 at the zoo, caused a global sensation when he survived rejection by his mother and attracted more than two million visitors to the zoo.

Knut the polar bear stands on his hind legs at his enclosure in the Berlin zoo (Zoologischer Garten) December 27, 2010.

Polar bear Knut interacts with six-year-old visitor Nane and her cushy polar bear "Knut" at the Tiergarten zoo in Berlin on October 19, 2010. German newspapers reported today that the world famous polar bear is being bullied by his fellow polar bears in their enclosure in the Berlin zoological garden.

Giovanna, a female polar bear on loan from the Munich zoo, dries herself off after taking a swim in the enclosure that she shares with Knut, the three-year-old world famous polar bear, at Berlin's Zoologischer Garten zoo January 8, 2010.

Knut, the 3-year-old polar bear (top) and Giovanna, a female polar bear on loan from the Munich zoo, play in their enclosure at Berlin's Zoologischer Garten zoo January 8, 2010.

Knut, the 3-year-old polar bear, rolls in the snow in his enclosure at Berlin's Zoologischer Garten zoo January 8, 2010.

Polar bear Knut plays in the snow at the Tiergarten zoo in Berlin on January 7, 2008. The bear caused a global sensation when he survived rejection by his mother and attracted more than two million visitors to the zoo.

Knut the polar bear yawns as he celebrates his first birthday at Berlin's Zoologischer Garten zoo in December 2007.

World famous polar bear Knut sticks out his tongue as he sits in his enclosure at the zoo in Berlin on July 11, 2008.

Knut, a three-month-old polar bear cub, plays with an old boot in his enclosure at Berlin's Zoologischer Garten zoo in March 2007.

Knut, the world's most famous polar bear, sticks out his tongue as he plays in his enclosure at the Berlin Zoologischer Garten zoo 02 October 2007. Knut caused a global sensation when he survived rejection by his mother nine months ago and attracted more than a million visitors to the zoo.

Knut, the world's most famous polar bear, plays with a toy as he takes a bath in his enclosure at Berlin's Zoologischer Garten zoo in October 2007.

Knut, the world's most famous polar bear, lays in his enclosure at the Berlin Zoologischer Garten zoo in December 2007, a few days before his first birthday.

Knut, the world's most famous polar bear, sits in his enclosure at the Berlin Zoologischer Garten zoo in October 2007, the day on which of the two milionth visitors came to see Knut, the zoo's biggest attraction.

The combo of pictures shows world famous polar bear Knut during his first outing at Berlin's Zoologischer Garten zoo March 23, 2007 (left) and on his first birthday December 5, 2007

In this combo of March 23, 2007 file pictures Knut the polar bear cub plays with a blanket during his first public appearance in the Berlin zoo. A Berlin zoo official says world-famous polar bear Knut has died. Bear keeper Heiner Kloes said that four-year-old Knut died Saturday afternoon March 19, 2011 while alone in his compound. He says the cause is not yet clear.

In this March 23, 2007 file photo, Knut, the polar bear cub, drinks water during its first public appearance in the Berlin zoo. A Berlin zoo official says world-famous polar bear Knut has died. Bear keeper Heiner Kloes said that four-year-old Knut died Saturday afternoon while alone in his compound. He says the cause is not yet clear

In this March 23, 2007 file photo, Knut, the polar bear cub, has its first public appearance with his keeper in the Berlin zoo. A Berlin zoo official says world-famous polar bear Knut has died. Bear keeper Heiner Kloes said that four-year-old Knut died Saturday afternoon while alone in his compound. He says the cause is not yet clear.

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

Knut, a three-month-old polar bear cub, appears to wave during his first outing at Berlin's Zoologischer Garten zoo 23 March 2007. The bear was born in December 2006 and rejected by his mother so that Doerflein was bottle-feeding him. Knut, who has become a global sensation, has his own video podcast from the zoo and a song has been written about him.

Knut the polar bear cub plays in his pen at Berlin's Zoologischer Garten zoo on April 3, 2007 in Berlin, Germany. Knut, a four-month-old polar bear was born on December 5, 2006 but was rejected soon after birth by his mother.