A case has to opened, yes? And the FBI director would have to appoint Mueller to investigate the case. He might appoint someone else. Mueller can't just investigate whatever he wants.
Under what authority would an FBI Director appoint Mueller?
The current FBI Director is Christopher Wray.
Robert Mueller does not work for the FBI but was seconded from law firm Wilmer Hale at the request of AAG Rosenstein to act as Special Counsel.
So, are you saying that Wray has the authority to appoint Mueller to investigate Clinton?