Efforts to contact Dr Palmer, who killed Zimbabwe's favourite lion Cecil in a trophy hunt, have been unsuccessful, a US wildlife agency has said.
The US Fish and Wildlife Service (USFWS) said in a statement yesterday it is deeply concerned about the killing of Cecil and is gathering information to assist Zimbabwean authorities in its investigation into the incident.
On twitter today, it said that efforts to contact American dentist Dr Walter Palmer, who shot the lion with a crossbow, have so far been unsuccessful.
USFWS is investigating the tragic killing of #CecilTheLion. Will go where facts lead. Efforts to contact Dr. Palmer so far unsuccessful.
The USFWS is investigating whether the killing of the beloved lion was part of a conspiracy to violate the Lacey Act, an American law against trading in wildlife that has been illegally killed, a source close the case said.
US police have launched an investigation into "terroristic" threats made against Minnesota dentist and trophy hunter Walter Palmer.
Palmer has become the target of global outrage after being outed as the killer of famous Zimbabwean lion Cecil.
Deputy Police Chief Mike Hartley said they were investigating a threat made over the phone to his officers.
A terroristic threats report related to this incident was taken by our police department yesterday and will be investigated like any other similar offence report.
An avid game hunter, Palmer has admitted to his part in the death of Cecil, who was lured from the protection of Hwange National Park before being shot - but said he believed the hunt was legal.
Palmer's dental surgery, River Bluff Dental, has been bombarded with hate mail and scathing online reviews, while protests have been held near the offices in Bloomington.
An 'ethical hunting' supporter has said "culling" is an important part of conservation work - but condemned those behind the killing of popular Zimbabwean lion Cecil.
Former Zimbabwe game guide Garth Hovell told ITV's Good Morning Britain that while African nations remained too poor to pay for culls themselves, trophy hunters would continue to wield power - and if people have concerns, he added, they need to "step up" and contribute financially.
US dentist Walter James Palmer paid $50,000 (£32,000) to kill the lion, which it later emerged had been lured from the protection of Hwenge National Park before being injured with a bow and arrow. He was shot dead with a rifle 40 hours later.
Mr Hovell said that culling was needed to prevent the spread of disease and to control populations, but said he had grave concerns about some of the practices being employed.
If the hundreds of thousands of people who have signed anti-hunting petitions even donated just $1 (64p) each, they could make a real difference.
These countries can't afford to pay for culls themselves, so they get wealthy people from around the world to come come and do it.
The problem that a lot of moderate people have and a lot of conservationists have is the way that hunting is being conducted - baiting animals out of the national parks, shooting at night - which is absolutely illegal in Zimbabwe, because you can't see what sex it is, and if they had seen a collar on Cecil hopefully they wouldn't have shot him.
Females are being shot, and pride males are being shot, which will lead to infanticide.
Professional hunter Theo Bronkhorst has been charged with organising the hunt, while game park owner Honest Ndlovu may also face charges.
Dozens of protesters gathered outside the practice of a suburban Minneapolis dentist who killed a protected lion in Zimbabwe.Read the full story ›
A professional hunter was charged today in Zimbabwe for failing to prevent the unlawful killing of Cecil the lion, whose death has caused widespread revulsion.
Theo Bronkhorst pleaded not guilty to "failing to supervise, control and take reasonable steps to prevent an unlawful hunt" in Hwange court and was bailed for $1,000 and ordered to surrender his passport.
He is due to return on August 5 for trial. If found guilty, he could be find $20,000 or jailed for up to 10 years.
American dentist, Walter James Palmer, who paid $50,000 to kill the oldest and most famous lion in Zimbabwe, said in a statement that he believed the hunt was legal.
Parks officials said that game park owner Honest Ndlovu, who is also accused of assisting Palmer, will first testify for the state before being charged.
A host of celebrities have taken to social media to speak out on the killing of well-known lion Cecil in Zimbabwe by a US dentist.Read the full story ›
These are the two men accused of illegal poaching after Zimbabwe's best loved lion Cecil was shot dead.
Game park owner Honest Ndlovu and hunter Theo Bronkhurst are accused of illegal poaching after it was alleged a US dentist paid them £32,000 so he could shoot the lion.
Cecil, who was a much-loved tourist attraction and the subject of a university study, was then skinned and beheaded.
Walter Palmer from Minneapolis admitted killing Cecil but claims he was told the hunt was legal. It is not yet known if he will also face charges over the killing.
Protesters have left stuffed animal toys outside the dental practice owned by the US hunter who killed Cecil the lion.
The River Duff Dental Practice in Bloomington, Minneapolis has been forced to close following the backlash against Walter Palmer.
Two men are due to appear in court in Zimbabwe charged with poaching offences over Cecil's death.
There are mounting calls to prosecute Palmer who claims he thought the hunt had been organised legally.
Two men are due to appear in court today over the killing of Cecil, Zimbabwe's most famous lion.
Hunter Theo Bronkhurst and private game park owner Honest Ndlovu are charged with poaching offences.
US dentist Walter Palmer wounded Cecil with a bow and arrow but the 13-year-old lion with not shot dead for another 40 hours.
He claims to his knowledge Bronkhurst and Ndlovu had organised the trip legally and said he had not been contacted by Zimbabwean authorities.
But Ndlovu was not allocated a lion on his hunting quota for this year, according to Zimbabwe Parks and Wildlife Management Authority.
If convicted, Bronkhurst and Ndlovu will be required to pay $20,000 in compensation and the court may impose an additional jail term.
Zimbabwe issues annual permits allowing foreign hunters to kill wildlife saying this allows it to raise money for conservation.
Angry animal lovers claim the US dentist who shot Cecil the lion is "not even a human being".
Christopher Flugge said: "If you're going to do something like that, be a man and actually go out in the wild where the animal has a chance to get some revenge against you.
"Any hunter that's worth a damn will probably find it incredibly disrespectful and repulsive. He's not even a human being as far as I'm concerned."
Jean Flugge added: "It upsets me that people can be so stupid to take a life like that, whether it's human or not."