When Knut, Berlin's famous baby polar bear, became a worldwide sensation last year, the spotlight also fell on Thomas Dörflein, his round-the-clock keeper assigned to raise the animal by hand after Knut's mother abandoned her offspring in December 2006.
And now Dörflein, 44, who played the role of Knut's surrogate parent, has died.
In a statement, Berlin Zoo director Bernhard Blaszkiewitz said he was deeply dismayed by Dörflein's sudden death.
"We've lost a longtime colleague and friend, and our thoughts and prayers are with his family," Blaszkiewitz said.
Berlin police, when contacted by ABC News, confirmed that Dörflein was found dead Monday but said they had no information on the cause of death.
The news about his passing has made headlines across Germany.
Today's edition of the tabloid Bild Zeitung calls his death "tragic" and writes that "a good heart has stopped beating."
Dörflein, who visited Knut Saturday, had taken Monday off for a routine doctor's appointment, according to the paper.
Dörflein was at a friend's home when he suddenly began grasping at his heart. He apparently collapsed and became unconscious, the tabloid reported. Though the friend called 112 (the emergency number in Germany) and emergency doctors arrived at the friend's apartment within minutes, they were unable to resuscitate him.
His body was taken to a morgue, pending an investigation into the cause of his death.
Dörflein gained fame in Germany and beyond when he became Knut's ever-present handler. He nursed the young polar bear, feeding him every three hours during the first couple of months, and even had a mattress next to the box where Knut slept.
He was at the bear's side when Knut made his first public appearance in March 2007, and the two of became inseparable during the following months.
Dörflein became almost as famous as Knut because they were seen playing together and were photographed by thousands of visitors who flocked to see the bear's daily appearance at his outdoor enclosure.
The Berlin Zoo saw a dramatic increase of visitors and is said to have made a few extra millions in profits because of Knut, who has his own feature-length film, "Cute Knut," along with a blog and a TV show and licensing deals for all kinds of Knut products, including stuffed animals, T-shirts, mugs and DVDs.
The daily "Knut & Thomas" performance ended in July 2007, when the zoo director ruled that the bear had grown too large for Dörflein to safely play with.
Knut had a hard time coping in the weeks after the separation. He was heard howling plaintively, apparently missing his keeper.
Knut has since grown into a pretty big bear. He weighs 265 pounds and lives on his own at a compound at the Berlin Zoo. Zoo officials are in touch with other international zoos to find him a partner to mate, but no decision has been made yet.
Dörflein was awarded Berlin's Medal of Merit for his service to the city -- and for taking care of Knut.
The zookeeper, who had worked for Berlin Zoo for more than 25 years, was a Berlin native. He reportedly leaves behind two grown children. He lived with his girlfriend and her young son.
"Thomas has a place in the hearts and minds of all Berliners for his rather unique, unselfish care and dedication that he has shown in bringing up Knut," said Blaszkiewitz, the zoo's director.