The U.S. Fish and Wildlife Service wants to talk to Walter Palmer. But it can't find him.

Investigators for the service have knocked on the front door of Palmer's house, stopped by his dental office, called his telephone numbers and filled his inbox with emails. Palmer, a hunting enthusiast who illegally killed a rare African lion in Zimbabwe early this month, couldn't be lured out of hiding.

"I'm sure he knows" the government is looking for him, said Ed Grace, chief of law enforcement for the U.S. Fish and Wildlife Service. "We've made repeated attempts to try and get in contact with him."

Palmer, a dentist in Minnesota, has fallen off the radar since Tuesday, when reports first surfaced identifying him as the hunter of Cecil the lion, a celebrity in Zimbabwe. In one of his only public statements, Palmer said Tuesday "I deeply regret" killing "a known, local favorite" and that he relied on local guides. He said he was led to believe the hunt was legal.

"I have not been contacted by authorities in Zimbabwe or in the U.S. about this situation, but will assist them in any inquiries they may have," Palmer said at the time.

Palmer has had plenty of time since to contact U.S. authorities, Grace said. He should know how to reach the agency "because we convicted him for lying about a bear kill" in Wisconsin in 2009, Grace said. Federal officials want to get Palmer's version of what went wrong in a hunt that killed a rare lion with a black mane. Cecil was iconic in Zimbabwe, and his fame worth at least $100,000 annually in eco-tourism, wildlife officials said.

Palmer's actions could have violated the U.S. Lacey Act, a conservation law meant to shield animals from harm. The act, tied to a United Nations treaty for the protection of animals, governs the actions of Americans who violate the laws of foreign governments.

Grace also said the State Department is looking into the matter in Africa. Officials at the State Department did not respond to a request to confirm that information. The three agencies often work closely to investigate crimes against wildlife, often involving the poaching of elephants and rhinoceros throughout southern Africa.

Most recently, they combined forces to investigate Out of Africa Adventurous Safaris in Montgomery, Alabama, which led to charges last year for Lacey Act violations, conspiracy, mail fraud, selling illegal rhinoceros hunts not sanctioned by the government and illegally trading rhinoceros horn, which can bring up to $45,000 per pound.

Zimbabwe officials have said the hunt that brought down its famed lion was illegal, regardless of the $50,000 Palmer paid for a permit.

Zimbabwean authorities charged professional hunter Theo Bronkhorst in Cecil's death; he was released Wednesday on $1,000 bail,The Associated Press reported. Landowner Honest Trymore Ndlovu has also been named by the country's parks service as being complicit in Cecil's murder, but his lawyer told the AP that Ndlovu has not been charged and was released.

Earlier in the week, Zimbabwean officials said Palmer could face poaching charges as well.

Safari Club International, which works with foreign governments to arrange legal big game hunts, suspended Palmer's membership. The Dallas Safari Club, which came under fire last year for auctioning off a permit for a hunt that ultimately led to the killing of a bull rhinoceros, disavowed Cecil's slaughter and called for a federal investigation.

Palmer has become the focus of an international firestorm as he has been vilified and threatened on the Internet. PETA has called for him to be hanged.

The vitriol even landed on the doorstep of Palmer's Minnesota dental practice, where protesters assembled to call for his extradition. Stuffed animals sit at the doorstep and a sign saying "Rot in hell" has been posted on the door.

Palmer has issued a letter to his patients, which was obtained by the Star Tribune and KSTP-TV.

"The media interest in this matter — along with a substantial number of comments and calls from people who are angered by this situation and by the practice of hunting in general — has disrupted our business and our ability to see our patients," Palmer wrote.

He added that the practice will refer patients with immediate needs to other dentists for now and "we will do our best to resume normal operations as soon as possible."

Zimbabwean officials believe Cecil was killed on private land on July 1. The 13-year-old male was one of the continent's most famous lions and lived on the Hwange National Park.

The non-governmental Zimbabwe Conservation Task Force has alleged that the hunters purposefully lured Cecil out of the park.

"They went hunting at night with a spotlight and they spotted Cecil," the group said in a statement. "They tied a dead animal to their vehicle to lure Cecil out of the park and they scented an area about half a kilometer from the park. Mr. Palmer shot Cecil with a bow and arrow but this shot didn't kill him.

"They tracked him down and found him 40 hours later when they shot him with a gun. They found that he was fitted with a GPS collar because he was being studied by the Hwange Lion Research, funded by Oxford University so they tried to destroy the collar but failed because it was found."

Palmer, an avid big-game hunter, has run afoul of the law before. He pleaded guilty in 2008 to making a false statement to U.S. wildlife officials in reference to the location of a black bear killing in Wisconsin.