In the United States, a grand jury consists of 16 to 23 people. Grand juries convene for a period of one month up to one year. The grand jury proceedings are held in private; the suspected criminal actor is usually not present at the proceedings.
The grand jury acts as an investigative body, acting independently of either prosecuting attorney or judge. 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 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.
The grand jury assesses whether there is adequate basis for bringing a criminal charge against a suspect. The grand jury is “a kind of buffer or referee between the Government and the people.” United States v. Williams, 504 U.S. 36, 37 (1992).