There are two (or three, depending on how you count the prosecutor) check points.
The first check point is the investigator. The threshold for taking action by this entity is "probable cause." This threshold is used to justify arrest, and to bother the prosecutor. Arrest is discretionary, but the FBI has that power.
If there is no probable cause, the case stops right there. It never gets past the investigator.
The next check point is the prosecutor. When the investigator finds probable cause, the case is turned over to the prosecutor. The prosecutor can decide to engage a grand jury, or not, for various and sundry reasons.
Comey can kill this with a fining of "no probable cause a crime has been committed." The prosecutors are totally out of that picture.
His only other option is to find probable cause a crime was committed, at which point the prosecutors are on the hot seat.
the real issue is the misuse of classified government information and its mishandling. the IG report indicates that personal server simply was not authorized. The email reports we have received show that the highest classified material went through that server. we know that it is to be protected and that it is to be protected whether one knows it is classified or not. that suggests there is no way that Hillary Clinton is not guilty of mishandling classified National information.
The truth is what it is. Politics trumps ltruth in today’s America. Comey is compromised. It’s a corrupt system.