This article by Mark Ames explains the history much more thoroughly, I highly recommend reading it.
In the late 60s through the early 70s a congressional committee led by Otis G. Pike investigated the FBI, CIA, and NSA. The committee spent years interviewing, issuing court subpoenas, asking hard questions about the dealings, efficacy, and legitimacy of these organizations. Their findings were very similar to what we see the NSA doing today: unconstitutional bulk data collection without warrants, lots of waste, no proof of efficacy, no oversight, … The report was eventually blocked from public release by the Ford Administration but parts of it were leaked to the press in spite.
The sad thing is that these findings (and other bad political news of the time) drove the public away from politics in general. The agencies (and the government) spun the committee and it’s report in the most negative way they could, causing the public to sigh, move on and wonder if any of it was true.
Edward Snowden has mentioned that his biggest fear in releasing the classified documents is that nothing would happen. This is still very possible as nothing happened the last time information like this was brought to light.