Ombudsman

An official appointed to investigate individuals’ complaints against maladministration, especially that of public authorities, acting as an independent intermediary between the public and the government.

Example: The Citizenship and Immigration Services Ombudsman, a federal office within the Department of Homeland Security, takes complaints from the public about USCIS processing problems and reports them to Congress and the agency.

Common misconception: An ombudsman has the legal power to force an agency to change its decision. An ombudsman typically has no binding enforcement power; their authority relies on investigation, mediation, and the power of public recommendation and reporting.

Sources: https://www.cisombudsman.org/ — Citizenship and Immigration Services Ombudsman (DHS official site): an official example of the office investigating complaints against an agency; https://www.britannica.com/ — encyclopedia fallback: "ombudsman" as a general concept has no U.S. federal definition