A grand jury has many differences and similarities with a trial jury. The grand jury is generally much larger, often comprised of at least 23 people. Like a petit jury, a grand jury is made up of regular citizens who respond to a jury summons.
The grand jury usually meets once a week for many months or even a year. This schedule is different from a petit jury, which usually meets every court day until a trial is over.
The grand jury is an investigative body that can issue subpoenas for witnesses or documents. The evidence is generally presented by prosecutors. The grand jury reviews the evidence and determines whether a person should be charged with a crime, or indicted.
A person being investigated, called "a target" of a grand jury, is not allowed to have an attorney present evidence to the grand jury. Neither the target nor the target's attorney may be present in the grand jury proceedings unless subpoenaed (or ordered to testify).
A target, if called to testify, or any other witness before the grand jury, may "plead the Fifth," or invoke their right to remain silent. A person may not later be punished for remaining silent because it is a constitutional right. Grand jury proceedings are kept secret until someone is charged. It is therefore possible a target may not know he or she is being investigated.
Ostensibly, the grand jury's purpose is to prevent overzealous prosecutors from charging people with crimes without sufficient evidence. However, in reality, grand juries usually follow the prosecutor's lead and do whatever the prosecutor suggests.
All federal criminal charges are made through a grand jury. Many states use grand juries as well. States are not required to use a grand jury. States may use other means of "checking" the prosecutor's exercise of charging discretion. For example, in California, prosecutors can use a grand jury, but they usually use a preliminary hearing instead.
In a preliminary hearing, the prosecutor presents evidence before a judge, who must decide whether there is probable cause to believe the defendant committed the charged crimes. A defendant may cross-examine witnesses and present evidence at a preliminary hearing, which is different than a grand jury.
The role of the grand jury is written into the U.S. Consitution. The Fifth Amendment states in part, "No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a grand jury."
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