This 2010 presentation and speaking notes from the NSA’s Office of Legal Counsel explains the legal authorities governing the agency’s computer network operations (CNO) activities: see the New York Times article Hunting for Hackers, N.S.A. Secretly Expands Internet Spying at U.S. Border, 4 June 2015.
This NSA memo, dated 3 January 2011, draws analysts’ attention to a rule change, allowing “contact chaining, and other analysis, from and through any selector, irrespective of nationality and location, in order to follow or discover valid foreign intelligence targets”: see the New York Times article N.S.A. Gathers Data on Social Connections of U.S. Citizens, 28 September 2013.
This undated image, taken from NSA training materials, shows the role played by Executive Order 12333 in authorising the agency’s activities: see the Washington Post article Privacy watchdog’s next target: the least-known but biggest aspect of NSA surveillance, 23 July 2014.
This 2007 US Department of Justice memo argues that analysis of US metadata is “consistent with” the Fourth Amendment, FISA and the electronic surveillance provisions of the United States Code [the Code of Laws of the United States of America] because the data is already in the NSA’s databases: see the Guardian article NSA collected US email records in bulk for more than two years under Obama, 27 June 2013.