Late Thursday afternoon, though, the agency's Office of Law Enforcement was contacted by somebody on Palmer's behalf. "The Service's investigation is ongoing and appreciates that Dr. Palmer's representative reached out," the agency said in a statement Friday.
If and when he materializes, Palmer could face an extradition request from officials in Zimbabwe, who have signaled a request to pursue one.
"We are appealing to the responsible authorities for his extradition to Zimbabwe so that he can be held accountable for his illegal action," Zimbabwe's environment minister Oppah Muchinguri said at a news conference on Friday, according to Reuters.
As of Friday morning, U.S. officials said they had not received an official extradition request.
Muchinguri said Palmer slipped away before government officials realized what had happened to the country's most famous lion.
"It was too late to apprehend the foreign poacher because he had already absconded to his country of origin," Muchinguri said, according to Reuters.
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 had plenty of time since to contact U.S. authorities, Ed Grace, chief of law enforcement for the U.S. Fish and Wildlife Service, told The Washington Post on Thursday. And 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, 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, Ala., 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.
As Zimbabwean officials called for Palmer's extradition, a petition urging the Obama administration to turn Palmer over blew past the threshold required for an official White House response, CNN reported.
More than 160,000 people have signed the whitehouse.gov petition, which urges Secretary of State John Kerry and Attorney General Loretta Lynch "to fully cooperate with the Zimbabwe authorities and to extradite Walter Palmer promptly at the Zimbabwe government's request."
As CNN noted, the petition needed 100,000 signatures by Aug. 27 to get a White House response.
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 slaughter, 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.
"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 in 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.
This post, originally published July 30, has been updated.