During a security clearance background investigation, verification of naturalization will primarily be done by:

Verification of naturalization during a security clearance background investigation is primarily done by checking records of the appropriate U.S. district courts. This is because U.S. district courts have jurisdiction over naturalization ceremonies and maintain official records of citizenship proceedings. These records provide the definitive legal documentation that confirms an individual's status as a U.S. citizen through naturalization.

While the State Department and FBI maintain various records that may relate to immigration and citizenship, they do not specifically hold the final and authoritative records regarding an individual's naturalization status in the same way that U.S. district courts do. The State Department focuses more on the issuance of passports and visas, while the FBI would be more concerned with criminal records and national security. Therefore, the primary source for verifying naturalization is indeed the appropriate U.S. district courts.