A grand jury's purpose is to investigate alleged crimes, examine evidence, and issue indictments if they believe that there is enough evidence for a trial to proceed. They are an impartial panel of citizens who must determine whether reasonable cause or probable cause to believe that a crime has been committed exists. The grand jury acts as a check on the prosecutorial power of the state.
The Fifth Amendment of the U.S. Constitution requires a grand jury to be empaneled in all "capital or infamous crimes" under federal jurisdiction. The Supreme Court has ruled that this requirement does not apply to states, so states are free to not use grand juries. In some jurisdictions, a prosecutor can issue charges by filing an information or accusation. A preliminary hearing before a judge with the defendant and his or her counsel is then held.
For example, in California, each county is required by law to impanel a body of 23 or 19 members, depending on county population, to serve for a term of one year. This body is mandated to investigate and report on both criminal and civil matters within the county. The following is from the Orange County, California website:
"The major functions of a grand jury are divided into criminal indictments and civil investigations. While both functions are executed by the same panel in Orange County, the civil investigation portion requires the majority of time.
The civil, or "watchdog" responsibilities of the grand jury include the examination of all aspects of county government, including special districts, to ensure the county is being governed honestly and efficiently and county monies are being handled appropriately. The grand jury is mandated by law to inquire into the conditions and management of public jails.
Investigations may be conducted on public agencies, the administration and affairs of any city within the county, or to examine books and records of redevelopment agencies. It is also appropriate for any private citizen, county official or county employee to present a written complaint to a grand jury for investigation.
During its term, the grand jury may select the government affairs which it wishes to investigate. These investigations are conducted by Grand Jury committees in cooperation with the departments and agencies of the county. Committees may ask for support and advice from Superior Court, District Attorney's office, County Counsel or outside consultants.
On the last day of the year of service, the grand jury is required by law to submit all final reports to the presiding judge of the Superior Court. These reports, with appropriate recommendations, include all studies and investigations conducted by the jury during its term. Each county entity investigated receives a copy of its individual report with a stipulation that response to findings/recommendations be made by the responsible governing body within 90 days. The public may find grand jury reports at most public libraries."