The prosecutors control Grand Juries, they decide the subject matter. You can indict a ham sandwich is you have the right prosecutor. Right now, they are all providing cover for BO.
While in some States that is true in other States District Attorneys have little or no control over the Grand Jury. In some cases the DA has been indicted by the Grand Jury for refusing to investigate other corrupt politicians.
http://dewdropwarriors.blogspot.com/2009_05_01_archive.html