Federal courts in Massachusetts and elsewhere use two kinds of juries: petit and grand. Petit juries hear evidence at trial and return a verdict of guilty or not guilty. The work of grand juries focuses on investigating federal crimes to determine whether probable cause exists that a certain person committed a certain crime. Its work is usually conducted in secret sessions, and its functioning is much less well understood by the general public.
The grand jury was developed in England beginning with its inclusion in Magna Carta in 1215. It migrated to America with many other legal institutions prior to 1776. When the Constitution and Bill of Rights were written in 1787, the grand jury was given a specific function by the Fifth Amendment: “no person shall be held to answer for a capital, or otherwise infamous crime, unless on the presentment or indictment of a Grand Jury…” The Supreme Court has ruled than an infamous crime is one punishable by one or more years of imprisonment.
Grand juries may be impaneled by a United States Attorney or a federal judge. A grand jury comprises 23 members, of which 16 constitute a quorum. The grand jury hears evidence presented to it by Assistant United States Attorneys. At the end of its deliberations on a particular matter, the grand jury will vote in secret about whether the evidence that it has heard demonstrates probable cause to believe that a crime has been committed.
Grand juries often seek testimony from persons who may know something about the matter under consideration but who are not suspects.
Source: Administrative Office of the United States Courts, “Handbook for Federal Grand Jurors,” accessed on March 31, 2018