28 C.F.R. § 90.39
Documentation of expenses retained on file by participating Tribes pursuant to sections 90.34 and 90.35 must be adequate for an audit. At a minimum, participating Tribes must retain the general accounting ledger and all supporting documents, including invoices, sales receipts, or other proof of expenses incurred for those expenses reimbursed by the Tribal Reimbursement Program. Such records must be retained for a period of three years from the end of the calendar year during which the participating Tribe sought reimbursement. All financial records pertinent to the Tribal Reimbursement Program, including the general accounting ledger and all supporting documents, are subject to agency review during the calendar year in which reimbursement is sought, during any audit, and for the three-year retention period.
28 C.F.R. §90.39