The Buchanan, Ingersoll & Rooney firm has notified the U.S. Justice Department of its intent to sue over the Eagle Herd Management Area roundup.
William Spriggs is lead counsel on the lawsuit expected to be filed Monday.
He says the U.S. Bureau of Land Management's roundups traumatize, injure and kill mustangs, and violate a 1971 law enacted by Congress to protect the horses.
He also represents California-based In Defense of Animals in a suit against the Interior Department over an ongoing roundup of about 2,500 mustangs in the Calico Mountain Complex north of Reno.
The BLM says the roundups are needed because of an over-population of mustangs.