Financial Audit: Bureau of the Fiscal Service's FY 2021 and FY 2020 Schedules of Federal Debt, Report to the Secretary of the Treasury [open pdf - 6MB]
From the Highlights: "GAO [Government Accountability Office] audits the consolidated financial statements of the U.S. government. Because of the significance of the federal debt to the government-wide financial statements, GAO audits Fiscal Service's Schedules of Federal Debt annually to determine whether, in all material respects, (1) the schedules are fairly presented and (2) Fiscal Service management maintained effective internal control over financial reporting relevant to the Schedule of Federal Debt. Further, GAO tests compliance with selected provisions of applicable laws, regulations, contracts, and grant agreements related to the Schedule of Federal Debt."
Government Accountability Office: https://www.gao.gov/