My guess is that the FBI is just scavanging in Metter's dumpster to find something incriminating enough to squeeze Metter's cojones and make him plead.
That saves them from having to rely on Moskowitz to throw him under a bus, and we all know that tutoring Moskowitz to say the right thing on the stand might be a real ordeal since the guy can't write a sentence that would pass an end of grade test for a fifth grader.
At least that's my take on the whole thing. Personally, and I know this is just me, but I believe the FBI and the USDOJ already have enough evidence to send Metter away for life and they should just get on with it and crucify the guy.