The bodies were found Saturday on public land along the Nevada-California border in Washoe County, about 120 miles north of Reno. Authorities believe the animals had been dead for about two weeks.
Dearing said investigators haven't determined a motive so far but would consider the possibility the case stems from controversy following the government's announcement earlier this year of a plan to round up and relocate thousands of wild horses.
While horse advocates brand the plan as inhumane and unnecessary, some Westerners view mustangs as a menace and think they should be euthanized instead.
"I'm sure that will be a consideration in the investigation," Dearing said. "Did it have something to do with what's going on? It's a very emotional issue.
"I wouldn't want to speculate on the motive," Dearing said. "It could have been a few good ol' boys whooping it up. We don't know."
Last year, the Interior Department warned that slaughtering some of the 69,000 wild horses and burros under federal control might be necessary to halt the rising costs of maintaining them. But Interior Secretary Ken Salazar has said his plan to relocate as many as 25,000 wild horses from Western rangelands to pastures in the Midwest and East would avoid that.
The horses' bodies were spotted by a helicopter pilot involved in a mustang roundup in the BLM's Buckhorn Herd Management Area. The BLM routinely removes what it considers excess horses from the range and takes them to government-funded holding facilities.
The discovery was made two days before a federal advisory board gathered near Reno to discuss implementation of Salazar's plan.
BLM spokesman Jeff Fontana said an agency special agent collected evidence at the scene Thursday, and an investigation continues.
Wild horses are protected under the 1971 Wild and Free-Roaming Horses and Burro Act, which prohibits harassing, capturing or killing the animals.
Violations are punishable by a fine of up to $2,000, a year in prison, or both.