Historians date the grand jury to King Henry II's Assize of Clarendon in 1166. That ancient ancestor was markedly different from its American descendants. The Grand Assize, as it was known, was comprised of local gentry, relying on personal knowledge and local rumor to report alleged cases of misconduct. Today's grand jury?surviving in America, but since 1933 abolished in England?normally considers events and people unknown to the grand jurors, who receive fairly formal testimony and other EVIDENCE, presented by prosecutors to decide whether or not alleged wrongdoers ought to be indicted.
Between 1166 and 1791, when the American BILL OF RIGHTS was adopted, the grand jury had come to be viewed as a safeguard for the people rather than an investigative arm of the executive. This is reflected in the portion of the Fifth Amendment that says: "No person shall be held to answer for a capital, or otherwise infamous crime, unless on a PRESENTMENT or INDICTMENT of a Grand Jury, except in cases arising in the land or naval forces, or in the Militia, when in actual service in time of War or public danger."
This means that nobody outside the armed forces may be put to trial for a serious federal crime unless a grand jury has heard enough evidence to satisfy it that there is PROBABLE CAUSE (enough evidence on the prosecution side, largely or wholly ignoring what the defendant may show, to make it reasonable) to issue an indictment. The good sense of the safeguard is the realization that "merely" being brought to trial can be an agonizing, expensive, destructive experience. In this light, the grand jury stands as a shield against arbitrary or wicked or careless prosecutors bringing people to trial on insufficient or improper grounds.
In modern times, this role as bulwark retains an exceedingly limited reality. As a practical matter, grand juries, especially in the busy urban settings where they do the bulk of their work, function largely as the investigative and indicting arms of prosecutorial officials. There could be no other feasible or acceptable way for them to operate. The detection of crime, the decision to investigate, the judgment as to where prosecution resources should be invested are no longer, if they ever were, subjects suitable for amateur, part-time management. Inevitably, then, grand jurors work almost entirely under the guidance and effective control of prosecutors. They consider cases brought to them by the government's lawyers. They tend almost always to indict when they are advised to indict, and not otherwise.
Although this quality of "rubber stamp" is markedly unlike the constitutional ideal, there is no agreeable alternative if we are to keep the grand jury as a body of lay citizens. The grand jury is a potent instrument...