First: this leaker simply mailed a packet to a random foreign government address which allowed usps to flag it?! Otherwise seems pretty shot in the dark they found it. 1234 Russia KGB, Russia ;)

Second: the FBI either got the cooperation of said unknown country, or was able to somehow make a signal looking like it originated from their embassy (maybe as comical as we'll fly our countries flag, which happens to always be flying). Maybe it's an ally?

Third: this person had a good amount of access? It sounds like schematics/info on the small nuclear reactors in the subs? Maybe that's not super valuable info?

So much I want to know! Would make a good dramedy someday if it's as dumb as it sounds.