How do we know that this is what has really happened? On the surface this seems to be possibly a case of parallel construction, in which FBI found the emails on

How do we know that this is what has really happened? On the surface this seems to be possibly a case of parallel construction, in which FBI found the emails on TorMail first and then asked for a warrant for the GMail mailbox to get enough evidence to get a warrant for the TorMail one. Is there a reason why this is impossible or improbable?