A grand jury does not determine guilt or innocence, but whether there is probable cause to believe that a crime was committed. The evidence is normally presented by an attorney from the States Attorney's Office.  The grand jury must determine from this evidence whether a person should have formal charges filed by the government.  If the grand jury finds probable cause, then it will return a written statement of the charge called an indictment.  

Grand jury proceedings are held in closed rooms, and the public is not allowed to witness it.  

The length of service is every Tuesday for three months.