Blair v. United States
In Blair v. United States, 250 U.S. 273, 282 (1919), the Court defined the vital investigatory function of the grand jury:
"It is a grand inquest, a body with powers of investigation and inquisition, the scope of whose inquiries is not to be limited narrowly by questions of propriety or forecasts of the probable result of the investigation, or by doubts whether any particular individual will be found properly subject to an accusation of crime. As has been said before, the identity of the offender, and the precise nature of the offense, if there be one, normally are developed at the conclusion of the grand jury's labors, not at the beginning. . . ."
Another passage from Blair pointed out the citizen's obligation to obey the process of the grand jury: "It is clearly recognized that the giving of testimony and the attendance upon court or grand jury in order to testify are public duties which every person within the jurisdiction of the Government is bound to perform upon being properly summoned." Id., at 281.