It isn't Comey's decision to release the statement. The FBI should gather the evidence, then present it to the AG, who then decides whether or not to prosecute.
Right now, because of his irrelevant letter, he has put the FBI direct in the center of a political mess.
They found 22 emails classified 'Top Secret' at which point it was decided they wouldn't be recommending prosecution, which was the same result as the AG.
What the heck could be worse than 22 Top Secret emails? More Top Secret emails? The precedent has already been set.
Has she murdered someone? Or requested the murder of someone? If so, then why not get that evidence all together, rather than dangle a carrot, and centralize your organization in a political mess, potentially destroying your own credibility, and that of the FBI.
Comey, with this single decision, has proven he is the wrong person to head up the FBI.