"In connection with an unrelated case, the FBI has learned of the existence of emails that appear to be pertinent to the investigation. I am writing to inform you that the investigative team briefed me on this yesterday, and I agreed that the FBI should take appropriate investigative steps designed to allow investigators to review these emails to determine whether they contain classified information, as well as to assess their importance to our investigation," he added.
OK, this is a big deal. Comey doesn't write a letter like this unless the determination has already been made, that it's serious, and the scale of the "new discoveries" is significant. And, probably, the discoveries catch Clinton or her people in flat-out lies of the sort to place them in legal jeopardy.
He doesn't write this letter for a couple of e-mails containing moderately confidential information.
Plus which, I believe he's desperate to reclaim some semblance of respectability for the FBI, which has been made to look like a political patsy by their behavior to date.