The American colonists liked the idea so much they enshrined it in the Fifth Amendment. The grand jury is really an arm of the court, which summons a pool of jurors, choosing and swearing in at least 16, but no more than 23 jurors. Grand juries are used only in cases where federal crimes may have been committed. When the grand jury convenes, the prosecutor presents it with a bill of indictment, listing the essential facts of the alleged crime committed by the targeted individual. As a result, it can listen to hearsay and other evidence that is normally inadmissible in court, such as evidence that may have been obtained illegally.
Testimony can be brief, or can take many months to complete.
In other ways, the work of the grand jury operates with the same informality of a normal jury. But members decide largely for themselves how to proceed, as long as they act as one group. The grand jury may call witnesses, and can compel them to appear or produce documents through a subpoena, although witnesses may refuse to answer questions by invoking a privilege against self-incrimination. A person who has been granted immunity from prosecution in exchange for testimony can be punished for contempt if he subsequently refuses to testify. And if witnesses lie in their testimony, they can be charged with perjury.