BERLIN — Berlin prosecutors said an autopsy Tuesday showed that the zookeeper who gained fame for hand-rearing the celebrity polar bear cub Knut died of a heart attack. The city's mayor paid tribute to him for writing Berlin history.

Thomas Doerflein, 44, was found dead in a Berlin apartment Monday. With the cause of death not immediately clear, prosecutors ordered an autopsy Tuesday.

A statement from Berlin prosecutors said the autopsy determined that Doerflein suffered a heart attack and that "other causes of death, in particular involvement by another party, are ruled out from a medical point of view."

Doerflein's death was front-page news in German newspapers Tuesday. Berlin's B.Z. tabloid, under the headline "Knut's daddy dead," dedicated 11 pages to the keeper, while the Berlin zoo set up an online condolence book. A few visitors left flowers outside Knut's enclosure.

Doerflein gained fame in Germany and beyond as the ever-present caretaker for Knut, abandoned by his mother after his birth in late 2006. Knut became a worldwide sensation when the Berlin Zoo decided to raise him by hand, and Doerflein was on hand at every stage of the bear's growth.

"Whether he wanted to or not, Thomas Doerflein wrote Berlin history in successfully raising Knut," Mayor Klaus Wowereit wrote in a message of condolences to Doerflein's mother. "In doing so, he always remained modest and down to earth."