The Grand Jury is responsible for, but not limited to, watching over other departments of County government. This includes the District Attorney's Office and the Sheriff. They also periodically file public reports with the court describing the results of their various investigations.
Any kind of felony case can be brought before the Criminal Grand Jury, but only the more serious cases are typically reviewed by them. Since its inception, the Criminal Grand Jury has investigated hundreds of felony cases in Contra Costa County.