BISMARCK, N.D. (AP) — North Dakota is challenging the federal government’s awarding of lucrative mineral rights under a Missouri River reservoir to the Mandan, Hidatsa and Arikara Nation, otherwise known as the Three Affiliated Tribes.

The state attorney general’s office said Friday that it filed notice with the U.S. District Court in Washington, D.C., that it plans to intervene in the tribes’ lawsuit against the federal government. It said its motion “preserves the state’s position while also allowing discussions to continue with the tribes and the United States to reach an amicable resolution.”

The Interior Department ruled in February that the tribes own the mineral rights, in what has been a long-running dispute. An Obama appointee affirmed the tribes’ ownership of the rights in 2017, but a Trump appointee ruled in the state’s favor in 2020 before the Biden administration scrapped that ruling last year.

At stake is an estimated $100 million in unpaid royalties held in trust and future payments certain to come from oil drilling beneath the river. The federal government dammed the river in the 1950s, flooding more than a tenth of the tribes’ 1,500-square-mile (3,885-square-kilometer) Fort Berthold Reservation and creating the reservoir, Lake Sakakawea.

Tim Purdon, an attorney for the tribes, said Monday that the Interior Department’s ruling didn’t resolve the tribes’ entire lawsuit, including their claim to unpaid royalties.

“The remaining claims are the financial accounting and payment of monies owed to the tribes for drilling under the river,” Purdon said.

The state argues that it assumed ownership of the riverbed when North Dakota became a state in 1889, citing cases in which the U.S. Supreme Court held that submerged lands were not reserved by the federal government.

The tribes, which reassumed control of the rights last month, point to three previous federal opinions dating back to the 1851 Treaty of Fort Laramie that they say confirm their ownership of the riverbed.

The tribes’ chairman, Mark Fox, said the state’s challenge was expected and that the tribes will fight it. He has previously condemned the state’s past efforts to intervene and said the tribes’ being granted ownership of the mineral rights corrected “a grave injustice.”

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