The FBI cannot prosecute neither can they "decline" a criminal case.  That's the function of the Department of Justice.  That was one of the problems with Comey.  He thought he ran the DOJ too.  Comey really wanted to prosecute Hillary? Really? He has a strange way of showing it.  Publically saying she has done nothing wrong.  Comey could prosecute Hillary?  I guess he runs the DOJ and the FBI.  Wow.  I didn't know that.