National Security Letters in Foreign Intelligence Investigations: Legal Background and Recent Amendments

"Five federal statutes authorize intelligence officials to request certain business record information in connection with national security investigations. The authority to issue these national security letters (NSLs) is comparable to the authority to issue administrative subpoenas. The USA PATRIOT Act expanded the authority under four of the NSL statutes and created the fifth. Thereafter, the authority has been reported to have been widely used. Prospects of its continued use dimmed, however, after two lower federal courts held the lack of judicial review and the absolute confidentiality requirements in one of the statutes rendered it constitutionally suspect." Source: Congressional Research Service, Library of Congress

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