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 notified the U.S. District Court in Washington, D.C., that it plans to intervene in the tribes’ lawsuit against the federal government.

The Interior Department ruled in February that the tribes own the rights, in what has been a long-running dispute. At stake is an estimated $100 million in unpaid royalties and future payments certain to come from oil drilling.

The government dammed the river in the 1950s, flooding more than a tenth of the tribes’ reservation and creating the reservoir.