A number of projects, some run by NSA and some by CIA, intercepted mail and electronic communications of US citizens in the US. These programs were, by 1975, discontinued as illegal without warrants. Various interception programs under the George W. Bush administration have been restarted, although the Administration has not explained them, and claims no warrants are needed under the doctrine of unitary authority.
CIA, and the rest of the intelligence community, receives product from thousands of NSA SIGINT programs. During this same period, CIA received extensive SIGINT on Southeast Asia and the Soviet bloc, and the rest of the world, and used this in preparing analytical products. Among these, for example, are nearly 200 National Intelligence Estimates on Southeast Asia, plus monthly summaries for Vietnam and specific studies relevant to military operations.
Among these were Project SHAMROCK and Project MINARET surveillance data on Americans, judged inappropriate by the Director of NSA and shut down by 1975. Much to the surprise of some, not everything CIA received from NSA involved impropriety.
Another program, HTLINGUAL intercepted physical mail, first simply recording the source and destination addresses on the envelope, and later surreptitiously opening and reading mail. This ran from 1952 to 1973.
Read more about this topic: CIA Activities In The United States