Some of the details are in the "Family Jewels," finally made public in 2007. The CIA not the FBI, my mistake there, would either arrange the post to be moved to a separate room or sneak mail out in briefcases or pockets.
Wow, I'm also very amazed at the description of the document:
> [T]his document consists of almost 700 pages of responses from CIA employees to a 1973 directive from Director of Central Intelligence James Schlesinger asking them to report activities they thought might be inconsistent with the Agency's charter.
I love the concept of just asking your employees to tell you everything they did that they're not supposed to, and they just write up a bunch of reports and send them back.
Also, fun is the warning about the PDF size, which feels a bit quaint:
> this file is 28MB, so please be patient while it downloads
https://www.cia.gov/library/readingroom/collection/family-je...