A federal irrigation project in the Klamath River basin required the United States Department of the Interior (defendant) to allocate scarce water rights among competing water users, including Indian tribes and non-tribal users. Under federal Indian law, the Interior department and its Bureau of Indian Affairs (defendant) were fiduciaries for Indian tribal interests. Both agencies consulted with tribes affected by the irrigation project. The tribes advocated for their interests during these consultations. The Klamath Water Users Protective Association (plaintiff), one of the competitors for water-allocation rights, filed federal Freedom of Information Act (FOIA) requests to obtain copies of agency and tribal documents relating to the consultations. The agencies withheld seven documents as intra-agency attorney work product and deliberative-process documents protected from disclosure by FOIA's Exemption 5. Tribal representatives prepared six of the seven documents. The association sued the agencies to obtain the withheld documents. A federal district court granted summary judgment for the agencies. The United States Court of Appeals for the Ninth Circuit reversed.