That would be the career federal employees at the involved agencies who might know something.
We pick out 4 or 5 of them who were in a position to observe and START IMPEACHMENT PROCEEDINGS AGAINST THEM.
I'm sure the whistleblowers already on the job know some good names. This is the way we work our way into the permanent bureaucracy in these agencies to get to the various career and appointed "Mr. Bigs" who need to be taken down to take us TO THE PRESIDENT and his running dog lackeys.
Just pick some GS 13/14 grade folks. Show them the instruments of torture with an impeachment hearing.
BTW, you probably can't prosecute Obama and Holder for these crimes until after they've been removed from office, so the way to get their attention and instill fear in them is to IMPEACH and REMOVE some low level people.
Besides, we can probably get enough Democrats in the Congress to go along with having Congress fire a GS 14 ~ I mean, like, does Congress care?