Project SHAMROCK, the sister project for Project MINARET, was an espionage exercise started in August 1945, which involved the accumulation of all telegraphic data entering into or exiting from the United States. The Armed Forces Security Agency (AFSA) and its successor, the National Security Agency (NSA), were given direct access to daily microfilm copies of all incoming, outgoing, and transiting telegrams via the Western Union and its associates RCA and ITT. NSA did the operational interception, and, if information that would be of interest to other intelligence agencies, the material was passed to them. "Intercepted messages were disseminated to the FBI, CIA, Secret Service, Bureau of Narcotics and Dangerous Drugs (BNDD), and the Department of Defense." No court authorized the operation and there were no warrants.
The precursor to the project according to Stephen Budiansky occurred in 1940, "In January 1940 the Army's adjutant general sent a letter to the president of RCA, David Sarnoff, asking if a Lieutenant Earle F. Cook might be assigned to the company..." Cook photographed all international commercial cablegrams. "The clandestine arrangement—almost certainly illegal—set a precedent..." Official wartime censorship began in Dec. 1940, when all cables were "turned over to the government for inspection." According to Tordella, "the collection program 'just ran on' ever since its beginning in World War II 'without a great deal of attention from anyone'..." Three major cable companies provided copies of all international telegrams passing through New York, Washington, and San Francisco. In the 1950s, "New Shamrock" tapped the links of 60–70 foreign embassies.
At the height of Project SHAMROCK, 150,000 messages were printed and analyzed by NSA personnel in a month.
In May 1975 however, Congressional critics began to investigate and expose the program. As a result, NSA director Lew Allen terminated it, on his own authority rather than that of other intelligence agencies. According to Budiansky, a 1977 US Department of Justice review concluded wiretap laws were violated, but "If the intelligence agencies possessed too much discretionary authority with too little accountability, that would seem to be a 35-year failing of Presidents and the Congress rather than the agencies or their personnel."
The testimony of both the representatives from the cable companies and of director Allen at the hearings prompted Senate Intelligence Committee chairman Senator Frank Church to conclude that Project SHAMROCK was "probably the largest government interception program affecting Americans ever undertaken."
One result of these investigations was the 1978 creation of the Foreign Intelligence Surveillance Act (FISA) which limited the powers of the NSA and put in place a process of warrants and judicial review. Another internal safeguard, was United States Signals Intelligence Directive 18 (USSID 18), an internal NSA and intelligence community set of procedures, originally issued in 1980.
USSID 18 was the general guideline for handling signal intelligence SIGINT inadvertently collected on US citizens, without a warrant, prior to the George W. Bush Administration. The post-Clinton era interpretations of FISA and USSID 18's principles assume that the executive branch has unitary authority for warrantless surveillance. This assertion came under congressional investigation as an apparent violation of FISA's intent.
- "Intelligence Activities and the Rights of Americans: National Security Agency Surveillance Affecting Americans". Icdc.com. Retrieved 2012-12-30.
- "The Origins of NSA (NSA.gov)". Archived from the original on March 18, 2004. Retrieved 2014-12-30.
- Senate Select Committee to Study Governmental Operations with Respect to Intelligence Activities (April 23, 1976). "Supplementary Detailed Staff Reports on Intelligence Activities and the Rights of Americans: National Security Agency Surveillance Affecting Americans".
- Budiansky, Stephen (2016). Code Warriors. New York: Alfred A. Knopf. pp. 23, 286–291. ISBN 9780385352666.
- National Security Agency (January 25, 2011). "U.S. Signal Intelligence Directive 18: Legal Compliance and Minimization Procedures" (PDF).
- Recollections from the Church Committee's Investigation of NSA: Unlucky SHAMROCK, CIA Center for the Study of Intelligence
- Unlucky SHAMROCK: The View From the Other Side, James G. Hudec, August 2000
- ECHELON: America's Secret Global Surveillance Network
- The NSA's Global Spying Network | by Patrick S. Poole
- The National Security Agency: The Secret Unveiled
- Development of Surveillance Technology & Risk of Abuse of Economic Information | PDF
- Schneier on Security: Project Shamrock
- House report on Project Minaret and Project Shamrock