@Mike Re: Perhaps?
So... you have a couple of things...
Yes the Feds do have access to the encrypted messages. There's a couple of ways that they can do this... And no, I don't know which way the do it...
Also FB would have to have a copy of the encrypted message. After all, they have to be able to transport and deliver the message.
What FB claims is that they can't decrypt it.
Note: After it became clear that Zuck and Company were considering selling access to user's data, it is not a far stretch to believe that they also have access to the keys. This way they can better target the user for ads.
Sort of like Google saying 'no human reads your gmail emails... (But their machine processing does. )
This is why I suspect FB doesn' t want to admit that it is feasible.
Of course it could be that they don't know the keys and that they are local to your keychain, but who knows and who cares? Why have a FB account?