A National Security Letter (NSL) is a form of administrative subpoena used by the United States Federal Bureau of Investigation and reportedly by other U.S. Government Agencies including the Central Intelligence Agency and the Department of Defense.
It is a demand letter issued to a particular entity or organization to turn over various record and data pertaining to individuals. They require no probable cause or judicial oversight. They also contain a gag order, preventing the recipient of the letter from disclosing that the letter was ever issued.
Declassified Documents
Federal Bureau of Investigation (FBI) / Office of the Inspector General Records
A Review of the FBI’s Use of National Security Letters: Assessment of Progress in Implementing Recommendations and Examination of Use in 2007 through 2009 [ 232 Pages, 134MB ]
Federal Bureau of Investigation (FBI) Records
Federal Bureau of Investigation (FBI) National Security Letters [1,188 Pages, 27.09MB ]
Office of the Secretary of Defense Documents Concerning National Security Letters [ 198 MB ]
Air Force Office of Special Investigations (AFOSI) Documents Concerning National Security Letters [ 48 MB ]
Naval Criminal Investigative Service (NCIS) Documents Concerning National Security Letters [ 385 MB ]
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