http://www.udayton.edu/~grandjur/faq/faq3.htm<snip>
In the federal system and in all but two of the states, grand juries are used to bring charges against persons who are believed to have committed crimes. In the federal system and in some states, they HAVE to be used to bring charges for felonies, which are the more serious crimes that normally carry a prison term for those who are convicted.
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If a prosecutor wants a grand jury to charge someone, the prosecutor reserves time with the grand jury and then presents evidence to them. In presenting the evidence, the prosecutor is trying to persuade the grand jurors that the people he or she wants to charge have committed certain crimes. The evidence can be almost anything--testimony from witnesses (including police officers or federal agents), documents, video recordings, tape recordings, the results of scientific tests (like DNA tests), photographs, etc.
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After they hear all the prosecutor's evidence, the jurors vote on a set of proposed charges--known as an "indictment"--which the prosecutor has drafted and gives to the grand jurors. If the grand jurors decide the evidence creates probable cause to believe the persons named in an indictment committed the crimes it charges them with, they vote to "return" the indictment, i.e., to charge the person with those crimes. Voting to return charges is usually known as "returning a true bill." If a grand jury votes to return a true bill, the indictment is valid and it initiates a criminal case against the people named as defendants in the indictment. A majority of the grand jurors must vote for an indictment in order to return a true bill. If a majority of the grand jurors don't think the prosecutor's evidence creates probable cause, they will vote not to return the indictment. When a majority of grand jurors vote not to return an indictment, this is known alternatively as "returning a bill of ignoramus" or "returning a no bill." If the grand jurors vote not to return an indictment, the indictment is not valid and no criminal case results.
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Lots of other information there.
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