Bloomington - A Minnesota dentist's killing of Zimbabwe's Cecil the lion just outside a national wildlife preserve has unleashed death threats and a global firestorm of hate messages on social media.About 200 people protested on Wednesday outside the suburban Minneapolis office of Walter Palmer, 55, calling for him to be extradited to Zimbabwe to face charges.Palmer, an avid big game hunter, said in a statement on Tuesday he regrets killing Zimbabwe's most famous lion on July 1. He said he had hired professional local guides who secured hunting permits and believed the hunt was legal.Cecil, a rare black-maned lion, was lured out of Hwange National Park using bait and was wounded with a bow and arrow, and not shot dead until 40 hours later.Cecil was fitted with a GPS collar for a research project by scientists from Oxford University and was one of the oldest and most famous lions in Zimbabwe.Palmer temporarily closed his office in Bloomington, Minnesota, on Tuesday as criticism grew of his killing of Cecil and negative business reviews flooded Google and Yelp.The lion's death has spawned half a dozen petitions on Change.org and calls by animal rights groups for US laws to protect big game animals and prevent hunters from bringing trophies back to the United States.Bloomington Police are investigating threats against Palmer, whose location is not known. Because many of the threats were made online, police are having difficulty determining their origins and credibility.Past problems with bear huntA small memorial of stuffed animals stood in the entry door to the building that houses Palmer's dental practice. Signs taped to the door said "Rot in Hell" and "Palmer there is a deep cavity waiting for you!"Demonstrators called for Palmer's arrest and asked people not to use his dental practice."Walter, you are a murderer, you are a terrorist," said Rachel Augusta, a Minneapolis resident and mentor coordinator at the Animal Rights Coalition, which organised the protest.Even Minnesota Governor Mark Dayton weighed in."It's an iconic lion," Dayton, a Democrat, told reporters. "To lure the animal out of the preserve, I don't understand how anybody thinks that's a sport. I just think it is horrible."Palmer, who has not been charged, has been under official scrutiny for his hunting in the past. He pleaded guilty in 2008 to a federal charge of lying to a US Fish & Wildlife agent about a black bear hunt in Wisconsin two years earlier. Palmer was sentenced to one year probation and fined $2 938.He had been accused of killing a bear outside his permitted zone and then hauling it back 64km inside to register it with authorities.ApologyIn a letter to patients reported by WCCO radio, he apologised for killing the well known lion."I don't often talk about hunting with my patients because it can be a divisive and emotionally charged topic," he said in the letter to patients. "I understand and respect that not everyone shares the same views on hunting."The US Fish and Wildlife Service is investigating whether any US laws were violated in the lion's killing and will assist Zimbabwe officials, said spokeswoman Vanessa Kauffman.Palmer said in his statement he had not been contacted by authorities in Zimbabwe or the United States and would assist in any inquiries. A spokesperson for the US Attorney's Office in Minnesota declined to comment.The attorney who represented Palmer in his Wisconsin case could not be reached immediately for comment.A professional hunter, Theo Bronkhorst, pleaded not guilty in a Zimbabwean court on Wednesday on charges of failing to prevent an unlawful hunt.Even US comedian Jimmy Kimmel weighed in on the lion hunt."I am honestly curious to know why a human being would feel compelled to do that. How is that fun?" Kimmel said on Tuesday night on his network television show.