BERLIN—The zookeeper who gained fame for hand-rearing the beloved polar bear Knut was found dead in his Berlin apartment Monday.

A spokesman for Berlin police said Thomas Doerflein was dead when authorities arrived at the apartment and that they had no information on the cause of death.

Doerflein was 44.

He gained fame in Germany and beyond as the ever-present caretaker for Knut, a polar bear cub abandoned by his mother in late 2006.

Knut became a worldwide sensation when the Berlin Zoo decided to raise him by hand, and Doerflein was there for every stage of the bear’s progress.

With his burly build, beard and ponytail, Doerflein was a distinctive figure at the side of the growing bear. He nursed young Knut in his arms behind closed doors and wrestled with him after the bear grew old enough to play.

When Knut made his public debut in March 2007, Doerflein was at his side. They started a daily performance for the thousands of visitors who flocked to see the bear at his outdoor enclosure.

But the “Knut show” ended in July of that year when the zoo’s director ruled that the bear had grown too large for Doerflein to frolic with in safety.

The boisterous bear now weighs more than 265 pounds.