"It is a response to that inquiry, which is being conducted right now by the Office of the Inspector General of the Interior Department," said Tom Gorey, BLM spokesman for the wild horse program in Washington, D.C.
Wild horse advocates said the rules amount to "window dressing" and won't keep large numbers of mustangs out of the hands of so-called kill buyers.
The inspector general is investigating what became of 1,777 horses sold since 2009 to Tom Davis. Wild horse advocates fear the animals were taken to Mexico for slaughter.
"He's the biggest buyer among all of our buyers over the years," Gorey said of Davis. Since 2005, when the BLM first allowed people to buy horses in bulk as opposed to adopting them, the agency has sold 5,400 animals, Gorey said.
An investigation of Davis' wild horse purchases was published by ProPublica in September.
Gorey said the inspector general is "looking into all aspects of the sales to Davis, including the whereabouts of the horses."
He said it's unknown when the investigation will be finished.
Wild horses are protected under federal law, and selling them for slaughter is illegal.
Davis signed agreements with the BLM promising none of the horses would be sold for slaughter, and he maintains he's done nothing wrong. Local authorities in Colorado also are investigating whether Davis violated brand inspection laws by shipping some horses out of state, according to published reports.
Under the new rules, sales of wild mustangs and burros will be limited to no more than four within a six-month period unless prior approval is obtained from a BLM assistant director. Buyers also must describe where they intend to keep the animals.
Requiring BLM approval for large sales won't protect mustangs, said Suzanne Roy, director of the American Wild Horse Preservation Campaign.
Roy said it is "outrageous" to put the decision of who gets more than four horses "in the hands of the very same BLM managers who were exposed as being responsible" for wild horses ending up with buyers like Davis. She added the new policy allows the BLM "to look the other way after six months."
Laura Leigh, an advocate with Wild Horse Education, said the BLM should rescind its policy that allows horse sales.
"Sales authority itself may be unconstitutional, and until it's repealed, our wild horses will remain vulnerable to sale into the slaughter pipeline," Leigh said.
About 37,000 wild horses roam Western states. About half are in Nevada.
The BLM, which is charged with managing wild horses and burros, has struggled to keep populations on the range in check. Hundreds if not thousands of horses are rounded up yearly and placed for adoption or sent to long-term holding pastures to live out their days.
Officials say there are now more horses in captivity—more than 45,000—than remain on the range.
From about 2005 until now, people who wanted 20 horses or more could buy them for $10 per horse and have them delivered by the BLM. Smaller numbers could be purchased for prices ranging from $25 to $75, said Heather Emmons, BLM spokeswoman in Reno.
"Most of the horses are older, and the majority would be sold for $25," she said.