Well, we know the NSA is behind some things, or else it wouldn't exist.
We also know this is an incredibly high profile case. It's in the interests of the DEA and FBI to lean on their contacts in the NSA, even.
That said, the FBI is very good at what it does, and finding this sort of thing isn't just what the FBI does, it's what any investigator does. (I have personal experience with this area that I can't disclose, but trust me, everyone knows how to link usernames between websites.)
We also know this is an incredibly high profile case. It's in the interests of the DEA and FBI to lean on their contacts in the NSA, even.
That said, the FBI is very good at what it does, and finding this sort of thing isn't just what the FBI does, it's what any investigator does. (I have personal experience with this area that I can't disclose, but trust me, everyone knows how to link usernames between websites.)