To add to the other comment a Grand Jury is also often made up of jurors called in using the prosecutor's private phone contacts. A few ex-cops and former work buddies. People also go to jail because of misuse of power by grand juries. There's a great documentary on Netflix but I can't remember its name right now. Suffice to say there aren't grand juries in any well working and fair justice system. It's abuse and/or theater 100% of the time.