A permit to hunt a black rhino in Namibia sold for US$350,000 at an auction in Dallas, with the proceeds going to protect the endangered animals.
But the sale still attracted strong protests from animal rights groups, who saw it as immoral conservation.
Steve Wagner, a spokesman for the Dallas Safari Club, which sponsored the closed-door event, confirmed the sale of the permit for a hunt, while declining to name the buyer.
But the auction caused anger among wildlife and animal rights groups, and the FBI said it was investigating death threats against members of the club.
About 40 protesters gathered on Saturday evening outside the convention centre where the auction took place. The licence allows for the killing of a single, post-breeding bull, with Namibian wildlife officials on hand for the hunt to make sure an appropriate animal is selected.
"Biologists in Namibia were hopeful that a US-based auction would produce a record amount for rhino conservation, and that's what happened," said the club's Executive Director Ben Carter.
He added: "These bulls no longer contribute to the growth of the population and are in a lot of ways detrimental to the growth of the population because black rhinos are very aggressive and territorial.
"In many cases, they will kill younger, non-breeding bulls and have been known to kill calves and cows."
More than 75,000 people signed an online petition at www.causes.com to stop the sale, saying black rhinos cannot be protected if they are allowed to be killed. FBI spokeswoman Katherine Chaumont said the agency was reviewing multiple threats against the club.
Carter said about a dozen threatening e-mails were posted on the group's website. Other messages were left with club sponsors criticising support for the organisation.
There are about 25,000 rhinos in Africa - 20,000 white and 5,000 black - with the majority in South Africa. Namibia is one of the leading habitats after that. Both countries allow for a few, carefully regulated hunts under internationally approved guidelines each year with proceeds going to fund conservation.
Officials from the Humane Society and the International Fund for Animal Welfare have said that while culling can be appropriate in abundant animal populations, all black rhinos should be protected, given their endangered status.
Rhino protection has grown more expensive in the last few years due to a surge in poaching fuelled by international crime syndicates to feed demand in Asia, where horn is used as a traditional medicine and sold at prices higher than gold.
Namibia is less hit by rhino poaching than its neighbour, South Africa, with only 10 killed since 2006, according to the international wildlife trade monitoring network Traffic.
Nearly 950 rhino were killed by poachers in South Africa last year, said the government.
Additional reporting by Associated Press