@InfraBlue,
You're looking in the wrong place. Actually it is the Attorney General who has the most control over these investigations, including what the FBI is allowed to do.
This was made clear by the FBI's numerous complaints that Loretta Lynch refused to authorize the formation of grand juries and the issuance subpoenas in their investigations of Bill and Hillary Clinton, essentially tying their hand and effectively ending their investigations..
All FBI "investigations" aside, it is the Attorney General's call about what will be prosecuted after any investigation is complete (well, unless maybe that function is usurped by a Director like Comey, anyway).