Of those, Crow officials could not provide adequate invoices and other documentation to explain $7.7 million in expenses, investigators found.

Separately, the tribe kept $4.8 million intended for the water project contracts and used the money to pay "business expenses unrelated to the contracts." The report did not specify the nature of the unauthorized expenses.

Representatives of the Inspector General's office did not immediately return a request for further comment.

When the tribe was asked to return the money to a specially-designated water projects account, tribal officials "asked to be put on a repayment plan," according to the audit. Details on how the tribe will repay the money were still being worked out.

Former Tribal Chairman Darrin Old Coyote was in office for much of the period covered by the audit. He said Tuesday that when he departed in December 2016, his administration left behind documentation detailing its spending on the water projects.

Old Coyote said those reports were never submitted. He also said the Bureau of Reclamation had been involved at every stage of the process and was aware of all spending by the tribe.

"They approved the projects. Step by step, they were right with us," he said.