I'm not a lawyer, but I think it has to be more than just "bias." They have to demonstrate that an FBI official actually engaged in professional misconduct and/or criminal activity. I honestly don't think that will be difficult to prove. In fact, I suspect Mueller has delayed Flynn's sentencing at least twice since January because he knows damn well how the case is going to end.
Agreed—they have to engage is some overt act—of course the whole Russia investigation was as phony as a three dollar bill—so pursuing it looks like one big overt act to me.