BISMARCK, ND (AP) — North Dakota is challenging the federal government’s award of lucrative mining 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 it had filed a notice with the U.S. District Court in Washington, DC, saying it planned to intervene in the tribes’ lawsuit against the federal government. He said his motion “preserves the state’s position while allowing discussions to continue with the tribes and the United States to reach an amicable resolution.”

The Interior Ministry decided in February that the tribes owned the mineral rights, in what has been a long-running dispute. An Obama appointee asserted ownership of tribal rights in 2017, but a Trump appointee ruled in favor of the state in 2020 before the Biden administration reversed that decision last year.

At stake is an estimated $100 million in unpaid royalties held in trust and future payments that will likely come from drilling for oil below the river. The federal government dammed the river in the 1950s, flooding more than a tenth of the tribes’ 1,500 square miles (3,885 square kilometers) Fort Berthold reservation and creating the reservoir, Lake Sakakawea.

Tim Purdon, a lawyer for the tribes, said on Monday that the Interior Department’s decision did not resolve the entirety of the tribes’ lawsuit, including their claim for unpaid royalties.

“The remaining claims are 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 ruled that submerged land was not reserved by the federal government.

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

Tribes Chairman Mark Fox said the challenge from the state was expected and the tribes will fight it. He has previously condemned past efforts by the state to intervene and said granting tribes ownership of mineral rights corrects “a grave injustice”.

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