BERLIN (Reuters) - Germany's celebrity polar bear cub Knut has received an anonymous death threat, causing alarm at Berlin Zoo on Thursday and prompting heightened security. (Advertisement) "We were contacted by the zoo about a (threatening) letter it had received," said a police spokeswoman, adding officers had investigated but they did not believe it was serious. Top-selling Bild newspaper said the zoo had received a hand-written fax from a suspected animal hater with the words: "Knut is dead! Thursday midday." It also said the zoo had trebled the number of minders to 15 responsible for the safety of media star Knut, who has been on newspaper front pages and news shows across Germany and around the world for weeks. "Death threat, police protection for Knut!" Bild wrote in bold letters on page one above a picture of the sad-looking polar bear cub peering out from behind a tree branch. Bild also published a picture of a security guard in civilian clothes guarding Knut from behind a rock. Thousands of fans, including many children, visit the zoo every day to see Knut on his twice daily excursions into an outside enclosure with his bearded keeper Thomas Doerflein. The keeper has slept in Knut's cage, played with him and fed him porridge since the cub's mother rejected him at birth. Knut, who now has his own brand, won global attention after questions were raised about hand-rearing polar bears. Some animal rights campaigners were interpreted calling for him to be put down rather than raised by humans. The ordeal for the four-and-a-half-month old cub comes just three days after officials cut short his daily show due to a fever. Vets reassured a concerned public he was suffering from teething pains and his swollen cheek was nothing to worry about. No one at the zoo was immediately available to comment.