The "DoJ... 3%" reference was in the 20C3 talk (2013) by RD & JA at
about 11 minutes in. I've got the video and don't recall where I got
it but it wasn't YouTube. The talk at that point was about the
perception of Tor and a slide was put up with four bullet points, the
first of which was "Doj's aborted study finding 3% bad content on the
Tor network".
Both RD & JA briefly commented on it but there's little more than what
Andrew has already conveyed. I'll be happy to transcribe that portion
here if you can't find the video.
IMO, though the 20C3 production values weren't the best, the talk
itself was one of the better ones. It's a keeper.