You've already got the how. Now for the really interesting question:
WHY did they think to try matching the finger prints found on a bag in Spain, to an American in the U.S.?
I seriously doubt we share those data with the Spanish government. Somebody had a reason to suspect him.
As I noted above, we know who this guy's been talking to. What this tells me is, the FBI knows about a lot more people than this one guy.