so basically they are a toll for the prosecutor and work for him/her, right?
can the prosecutor endict/charge if they say no?
Well, in PA, the grand jury doesn't issue indictments. The thing is, the standard of proof before the grand jury is much, much lower than what the prosecutor has to prove at trial. If the grand jury doesn't think there's even enough information to believe that the subject of the investigation committed a crime, then it's extremely unlikely that a prosecutor could prove charges beyond a reasonable doubt. So most of the time, they wouldn't file charges against someone after the grand jury said there isn't enough evidence - unless more evidence was found. Most prosecutors don't file cases they can't win. They'd want more evidence.
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