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.
Federal Bureau of Investigation (FBI) National Security Letters [1,188 Pages, 27.09MB ]