In 1976 and again in 1977, the Justice Department decided not to prosecute anyone for the CIA’s illegal surveillance and mail openings. The report issued in 1977 reveals the Justice Department’s highly flawed reasons, including claims that prosecution would not serve to prevent such questionable or outright illegal surveillance from happening again - ironically setting the stage for modern surveillance programs.
The 57 page report begins with a surprising assertion – the Justice Department disagreed with everyone about the mail opening program being illegal. According to the Justice Department, “it would be mistaken to suppose that it was always clearly perceived that the particular mail opening programs of the CIA were obviously illegal.” Saying that they disagreed with everyone is not hyperbole – even CIA admitted that the programs were illegal.
https://www.muckrock.com/news/archives/2017/apr/07/justice-department-refused-prosecute-cia/