Lack of full body and some headers mentioned in the DKIM-Signature headers makes it impossible to verify DKIM authenticity. Would (reddit) OP not cut out their Authentication-Results headers, we we would know how their MTA's anti-forgery mechanisms saw this alleged message.
But, assuming that what's on reddit is true, this is interesting. It looks like FBI attempting to discredit a researcher (which I doubt because this would be one of dumbest ways to do so) or maybe someone gained enough access to FBI's infra to at least bounce a message by their systems without it looking so (but earlier Received headers do not suggest that the message originated from outside the network).
EDIT: Another idea is that OP's systems may be so compromised already that someone simply created FBI-looking message on their system and it never touched network.
But, assuming that what's on reddit is true, this is interesting. It looks like FBI attempting to discredit a researcher (which I doubt because this would be one of dumbest ways to do so) or maybe someone gained enough access to FBI's infra to at least bounce a message by their systems without it looking so (but earlier Received headers do not suggest that the message originated from outside the network).
EDIT: Another idea is that OP's systems may be so compromised already that someone simply created FBI-looking message on their system and it never touched network.