In the federal system and certain states, prosecutors can initiate a criminal case against someone using a grand jury. A grand jury is a panel of citizens called for service just like a petit jury (also called a trial jury). But unlike petit juries that decide issues of guilt, grand juries decide if enough evidence exists to charge someone with a crime in the first place. When they do, they issue what's called an indictment (in-DITE-ment), which states the charges against an individual.
Federal prosecutors must use grand juries to bring felony charges against someone. While states are not required to use grand juries, many do in murder, capital, or other serious felony cases. A grand jury is just one way a prosecutor can initiate a case against someone. In instances where grand juries are not required, prosecutors can typically choose between using a grand jury or filing a criminal complaint on their own initiative.
Both petit and grand juries consist of randomly selected people who listen to evidence. Grand juries generally consist of 15 to 23 people who will consider whether the prosecutor's evidence supports charging the suspect with a crime. Petit juries are made up of six to 12 people who decide whether a defendant is guilty beyond a reasonable doubt. Petit juries must be unanimous in their decision to convict someone of a crime, whereas grand juries don't need to be unanimous to indict or charge someone.
Petit jurors serve the length of the particular trial, usually under ten days, unless it is a more serious or difficult case. Grand jurors serve for a longer period of time, often the length of a term of court, between six and 18 months. While a grand juror's commitment to serve can span months, a grand jury might convene only a few days each month.
In a grand jury proceeding, a prosecutor presents evidence and the grand jury then considers whether that evidence is enough to establish that the accused probably committed a crime. Grand jury proceedings differ from other criminal proceedings in that they are not adversarial. They are a one-sided affair, and neither the suspect nor a defense attorney is present. (The idea being that the suspect—upon becoming a defendant—will have the opportunity to present evidence at trial.)
Prosecutors can call witnesses and suspects to testify at a grand jury hearing. A subpoenaed witness could be a target of a grand jury investigation without knowing it. Prosecutors do not have to notify or warn witnesses of their investigations. Anything a witness says at a grand jury hearing could be used against them in a trial. Witnesses have the right to refuse to answer questions pursuant to the Fifth Amendment, so as to not incriminate themselves, but they cannot have a lawyer with them during the proceedings. (They could, however, ask to consult with their lawyer before answering a possibly self-incriminating question.)
If the grand jury concludes that probable cause exists based on the prosecutor's evidence, it issues an indictment—a formal accusation of criminal charges against the accused. If the grand jury doesn't indict, no charges are filed at that time. In such a case, a prosecutor can come back with more evidence and try to convince the same grand jury to indict. Or the prosecutor could choose to present the same evidence to a different grand jury in hopes of getting an indictment.
Part of the grand jury's duty is to act as a check on the prosecution and protect individuals from malicious or unfounded prosecutions. Grand jurors have the right to question witnesses and investigate the allegations. But, in practice, the influence of the prosecutor usually dominates the proceedings, and grand juries frequently indict based on the prosecution's evidence.
Sometimes prosecutors choose to pursue an indictment from a grand jury rather than filing a criminal complaint because, unlike other criminal proceedings, grand jury proceedings are secret and one-sided. When a prosecutor files a criminal complaint (rather than going through the grand jury process), the prosecutor must convince a judge in a public, preliminary hearing that they have enough evidence to convict. During this preliminary hearing, the defense can cross-examine the witnesses and see what evidence the prosecution has. This proceeding gives the defense a chance to preview the prosecution's case and better prepare a strong defense for trial. Whereas the secrecy of the grand jury proceedings gives prosecutors an advantage at trial by allowing the prosecutor a chance to test their witnesses and evidence before a jury without needing to reveal their arguments or evidence to the defense.
If you've been subpoenaed by a grand jury to testify as a witness, it's worth speaking to a criminal defense lawyer to make sure you are not putting yourself at risk. If you face charges based on a grand jury indictment, you'll want to find a criminal defense attorney to represent you right away.