This is a really good read, explaining the current problems FBI have with their illegal spying.
Even for a friendly FBI Agent to peruse these boards without a current signed "probable cause" warrant is highly illegal.
The FBI cannot appropriate powers that Congress did not explicitly delegate to them, and in any case, if its illegal for congress, the it's also illegal for any subordinate agencies.
This is what the Supreme Court V EPA ruling states quite clearly.
https://epublications.marquette.edu/cgi/viewcontent.cgi?article=1110&context=dissertations_mu