According to Cornell Law School’s Legal Information Institute, a grand jury is made up of between 16 to 23 people who are selected to convene for as long as one month up to a year. The suspected offenders are not present for the proceedings, which are also not open to the public.
A grand jury is separate from the judge and prosecutor. According to United States v. Williams, it acts as “a kind of buffer or referee between the Government and the people.”
“Criminal prosecutors present the case to the grand jury. The prosecutors attempt to establish probable cause to believe that a criminal offense has been committed,” the Legal Information Institute states. “The grand jury may request that the court compel further evidence, including witness testimony and subpoenas of documents. The grand jury is generally free to pursue its investigations unhindered by external influence or supervision.”
Potential people selected to sit on grand juries are selected in the same way trial jurors are chosen, via mailed calls to appear for jury duty.