Authorities at Berlin Zoo called in extra security on Thursday (April 19) after receiving an anonymous death threat against Germany's celebrity polar bear cub Knut. Police said they were contacted by the zoo about a threatening letter, which was investigated but the threat did not appear to be serious. Zoo managing director Gerald Uhlich told Reuters: "We were told that this happens a lot to prominent people and animals, that some rather crazy people send these kind of messages. We also were told not to take it all that seriously, so there does not to be a serious threat. But just to be on the safe side, we have taken all security measures possible," he said. Top-selling Bild newspaper said the zoo had received a hand-written fax saying: "Knut is dead! Thursday midday." It said the zoo had trebled, to 15, the number of guards responsible for the safety of media star Knut, who has been on the front page and news shows across Germany and around the world for weeks. "Death threat, police protection for Knut!" Bild said on its front page above a picture of the 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 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. Some visitors to the zoo were concerned that the danger to the bear might be real. Visitor Wolfgang Mueller said he could not understand the reason for the threat. "This is a madman," he told Reuters. "If he was to shoot Knut... I do not understand what goes on in the heads of people like that." And another visitor, Dieter Page, said he could not understand how anyone could threaten an animal like Knut. "I don't know what it is with people like that, we are talking about a baby animal here. I don't know what the point of the people is. I just do not get it," he said. Three days earlier, officials cut short his daily show because he had a a fever. Vets reassured a concerned public he was suffering from teething pains and his swollen cheek was nothing to worry about. Knut hit the headlines after questions were raised about hand-rearing polar bears. Some animal rights campaigners were objected to him being raised by humans. NP/AD