In the Patriot Reauthorization Act, enacted in 2006, Congress directed the Department of Justice (Department) Office of the Inspector General (OIG) to review "the effectiveness and use, including any improper or illegal use, of national security letters issued by the Department of Justice." The Act required the OIG to conduct reviews of the use of national security letters for two separate time periods. This report describes the results of the first OIG review of the FBI's use of national security letters (NSLs), covering calendar years (CY) 2003 through 2005.