48
Opinion of the Court
The grand jury's functional independence from the Judicial Branch is evident both in the scope of its power to investigate criminal wrongdoing and in the manner in which that power is exercised. "Unlike [a] [c]ourt, whose jurisdiction is predicated upon a specific case or controversy, the grand jury 'can investigate merely on suspicion that the law is being violated, or even because it wants assurance that it is not.' " United States v. R. Enterprises, Inc., 498 U. S. 292, 297 (1991) (quoting United States v. Morton Salt Co., 338 U. S. 632, 642-643 (1950)). It need not identify the offender it suspects, or even "the precise nature of the offense" it is investigating. Blair v. United States, 250 U. S. 273, 282 (1919). The grand jury requires no authorization from its constituting court to initiate an investigation, see Hale, supra, at 59-60, 65, nor does the prosecutor require leave of court to seek a grand jury indictment. And in its day-today functioning, the grand jury generally operates without the interference of a presiding judge. See Calandra, supra, at 343. It swears in its own witnesses, Fed. Rule Crim. Proc. 6(c), and deliberates in total secrecy, see United States v. Sells Engineering, Inc., 463 U. S. 418, 424-425 (1983). True, the grand jury cannot compel the appearance of
witnesses and the production of evidence, and must appeal to the court when such compulsion is required. See, e. g., Brown v. United States, 359 U. S. 41, 49 (1959). And the court will refuse to lend its assistance when the compulsion the grand jury seeks would override rights accorded by the Constitution, see, e. g., Gravel v. United States, 408 U. S. 606 (1972) (grand jury subpoena effectively qualified by order limiting questioning so as to preserve Speech or Debate Clause immunity), or even testimonial privileges recognized by the common law, see In re Grand Jury Investigation of Hugle, 754 F. 2d 863 (CA9 1985) (opinion of Kennedy, J.) (same with respect to privilege for confidential marital communications). Even in this setting, however, we have insisted that the grand jury remain "free to pursue its investi-
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