"The CIA conducted four mail opening programs within the United States, the longest of which lasted for twenty years. These programs resulted in the opening and photographing of nearly a quarter of a million items of correspondence, the vast majority of which were to or from American residents. While the programs were ostensibly conducted for foreign intelligence and counterintelligence purposes, one former high-ranking CIA official characterized the Agency's use of this technique as a 'shotgun' approach to intelligence collection; neither Congressmen, journalists, nor businessmen were immune from mail interception. With cooperation from the FBI, domestic 'dissidents' were directly targeted in one of the programs."
Supplementary Detailed Staff Reports on Intelligence Activities and the Rights of Americans - Book III (1976)