Spriggs has argued in several cases that the bureau's programme to manage the horses is executed in an illegal and unsustainable manner.
In his letter, Spriggs said he hoped Attorney General Eric Holder would ensure the bureau complied with the law and managed the wild horse population to guarantee their survival for years to come.
"Most egregiously, the bureau flouts the Wild Free-Roaming Horses and Burros Act through its systemic relocation of protected wild horses from their homes on designated Western ranges to long-term holding facilities located throughout the mid-Western United States," Spriggs said.
"Wild horses are relocated from where they are presently found and transferred to long-term facilities where wild horses do not currently exist."
Spriggs said the act stated clearly that horses should not be relocated to areas of public lands where they do not currently exist.
"Wild horses must be managed by the bureau where they are presently found, on their protected ranges."
Spriggs said the bureau's policy of "systemically ridding the West of wild horses by shipping them to the mid-West where they will be intensively managed for the rest of their natural lives is patently illegal per the provisions of the act".
He continued: "The bureau cannot claim that its long-term holding facilities are located in areas where wild horses have historically existed, or that its movement of the horses from Nevada to Oklahoma, Kansas, and/or South Dakota does not constitute a relocation."
Spriggs aruged that the Wild Horse and Burro Programme, as administered by the bureau, "is also a highly controversial expenditure of the government's money".
"I turn to you, as head of the Department of Justice and chief enforcer of the nation's laws, to please halt this gross abuse of power."