"National Security Letters (NSLs) are roughly comparable to administrative subpoenas. Various intelligence agencies use them to demand certain customer information from communications providers, financial institutions, and consumer credit reporting agencies under the Right to Financial Privacy Act, the Fair Credit Reporting Act, the National Security Act, and the Electronic Communications Privacy Act. The USA PATRIOT Act expanded NSL authority. Later reports of the Department of Justice's Inspector General indicated that (1) the FBI [Federal Bureau of Investigation] considered the expanded authority very useful; (2) after expansion the number of NSL requests increased dramatically; (3) the number of requests relating to Americans increased substantially; and (4) FBI use of NSL authority had sometimes failed to comply with statutory, Attorney General, or FBI policies. […] This report reprints the text of the five NSL statutes as they now appear and as they appeared prior to amendment by the USA PATRIOT Act (to which form they would be returned under S. 1125 and H.R. 1805). Related reports include CRS [Congressional Research Service] Report R40138, Amendments to the Foreign Intelligence Surveillance Act (FISA) Extended Until June 1, 2015, by Edward C. Liu, and CRS Report RL33320, National Security Letters in Foreign Intelligence Investigations: Legal Background and Recent Amendments, by Charles Doyle."
CRS Report for Congress, R41619