The FBI is investigating death threats made against members of the Dallas hunting club that intends to auction off a rare permit to kill an endangered black rhino, an FBI spokeswoman said Wednesday.

Katherine Chaumont said the agency is reviewing multiple threats against the Dallas Safari Club. The club on Saturday plans to auction a permit the African country of Namibia granted for the hunt. The group has said all proceeds will go toward rhino conservation efforts.

"The FBI is aware of the threats," Chaumont said. "If a violation of federal law is determined, additional action or investigation as necessary will take place."

The club's executive director, Ben Carter, said the threatening messages — about a dozen sent by email and posted on the group's website — appear to be orchestrated by people who oppose hunting. Other messages have been left with club sponsors criticizing support for the organization.

"I've had death threats on my family," Carter said. "We've had a number of death threats to our members and (threats about) what would happen if we sell the permit.

"Some crazy stuff," he said.

The auction is being held amid tightened security as part of the club's three-day annual convention in Dallas, which is expected to draw about 45,000 people. The group announced in October that it would auction the permit, one of only five offered annually by Namibia. The permit is also the first to be made available for purchase outside of that country.

An estimated 4,000 black rhinos remain in the wild, down from 70,000 in the 1960s. Nearly 1,800 are in Namibia, according to the safari club.

Poachers long have targeted all species of rhino, primarily for its horn, which is valuable on the international black market. Made of the protein keratin, the chief component in fingernails and hooves, the horn has been used in carvings and for medicinal purposes, mostly in Asia. The near-extinction of the species also has been attributed to habitat loss.

Carter said the permit could fetch $1 million. But organizers hope to at least break the previous high bid for one of the Namibia permits, which is $223,000.

Wildlife groups have criticized the promotion of a hunt targeting an endangered animal, but Carter said it's meant to cull aggressive rhinos in an effort to protect the larger herd. He said the Namibia hunt will focus on an older, nonbreeding male with a pattern of aggression toward other rhinos.

Carter said that wildlife experts say culling a herd is an acceptable habitat management practice.

"When you have the science and facts behind it, and people don't want to listen and just become emotional, you just wonder how people's brains work sometimes," he said.

Wayne Pacelle, president of the Humane Society of the United States, said culling a herd is acceptable for a population that's abundant, but not for a species on the federal endangered species list.

"We've had a standard for more than 40 years that you don't shoot an animal that's endangered," he said Wednesday.

Jeffrey Flocken, North American regional director of the Massachusetts-based International Fund for Animal Welfare, also said Wednesday that culling the herd is the wrong approach, given the limited number of black rhinos in existence. The better approach is to protect the rhino by establishing a secure habitat that welcomes the paying public to view the animal, he said.

"This auction is telling the world that an American will pay anything to kill their species," Flocken said. "This is, in fact, making a spectacle of killing an endangered species."