Grand juries are impaneled on a county-by-county basis. Each grand jury can only hear cases about events that happened in its own county. Grand jury proceedings in criminal issues are not open to the public.
The Constitution authorizes grand juries to issue indictments of suspects upon a finding of probable cause that the suspect committed an offense. Probable cause is simply enough evidence that would lead a person of ordinary caution or prudence to believe and conscientiously entertain a strong suspicion of the guilt of the accused.
The prosecutor presents the case to the grand jury using witnesses and other evidence and can even answer legal questions about the case for the grand jury. Once the evidence is heard, the grand jury deliberates in secret. Generally, grand juries consist of 19 citizens of the county. To return an indictment, a 19 member panel needs 12 members to vote for indictment.
In special cases, the prosecutor may tell defense counsel that the suspect has been indicted so counsel can arrange for a voluntary surrender to police. If the prosecutor does not tell the defense counsel of the indictment, the court will issue a bench warrant for the suspect's arrest.