If you have not read it yourself, here is the complete US vs Williams ruling. It is a long read but remember, knowledge is power.
http://www.law.cornell.edu/supremecourt/text/504/36
From that ruling:
"Respondent does not contend that the Fifth Amendment itself obliges the prosecutor to disclose substantial exculpatory evidence in his possession to the grand jury. Instead, building on our statement that the federal courts "may, within limits, formulate procedural rules not specifically required by the Constitution or the Congress," United States v. Hasting, 461 U.S. 499, 505, 103 S.Ct. 1974, 1978, 76 L.Ed.2d 96 (1983), he argues that imposition of the Tenth Circuit's disclosure rule is supported by the courts' "supervisory power." We think not. Hasting, and the cases that rely upon the principle it expresses, deal strictly with the courts' power to control their own procedures. See, e.g., Jencks v. United States, 353 U.S. 657, 667668, 77 S.Ct. 1007, 1013, 1 L.Ed.2d 1103 (1957); McNabb v. United States, 318 U.S. 332, 63 S.Ct. 608, 87 L.Ed. 819 (1943). That power has been applied not only to improve the truth-finding process of the trial, see, e.g., Mesarosh v. United States, 352 U.S. 1, 9-14, 77 S.Ct. 1, 5-8, 1 L.Ed.2d 1 (1956), but also to prevent parties from reaping benefit or incurring harm from violations of substantive or procedural rules (imposed by the Constitution or laws) governing matters apart from the trial itself, see, e.g., Weeks v. United States, 232 U.S. 383, 34 S.Ct. 341, 58 L.Ed. 652 (1914). Thus, Bank of Nova Scotia v. United States, 487 U.S. 250, 108 S.Ct. 2369, 101 L.Ed.2d 228 (1988), makes clear that the supervisory power can be used to dismiss an indictment because of misconduct before the grand jury, at least where that misconduct amounts to a violation of one of those "few, clear rules which were carefully drafted and approved by this Court and by Congress to ensure the integrity of the grand jury's functions," United States v. Mechanik, 475 U.S. 66, 74, 106 S.Ct. 938, 943, 89 L.Ed.2d 50 (1986) (O'CONNOR, J., concurring in judgment). 6
"We did not hold in Bank of Nova Scotia, however, that the courts' supervisory power could be used, not merely as a means of enforcing or vindicating legally compelled standards of prosecutorial conduct before the grand jury, but as a means of prescribing those standards of prosecutorial conduct in the first instance—just as it may be used as a means of establishing standards of prosecutorial conduct before the courts themselves. It is this latter exercise that respondent demands. Because the grand jury is an institution separate from the courts, over whose functioning the courts do not preside, we think it clear that, as a general matter at least, no such "supervisory" judicial authority exists, and that the disclosure rule applied here exceeded the Tenth Circuit's authority.
"Rooted in long centuries of Anglo-American history," Hannah v. Larche, 363 U.S. 420, 490, 80 S.Ct. 1502, 1544, 4 L.Ed.2d 1307 (1960) (Frankfurter, J., concurring in result), the grand jury is mentioned in the Bill of Rights, but not in the body of the Constitution. It has not been textually assigned, therefore, to any of the branches described in the first three Articles. It " 'is a constitutional fixture in its own right.' " United States v. Chanen, 549 F.2d 1306, 1312 (CA9 1977) (quoting Nixon v. Sirica, 159 U.S.App.D.C. 58, 70, n. 54, 487 F.2d 700, 712, n. 54 (1973)), cert. denied, 434 U.S. 825, 98 S.Ct. 72, 54 L.Ed.2d 83 (1977). In fact the whole theory of its function is that it belongs to no branch of the institutional government, serving as a kind of buffer or referee between the Government and the people. See Stirone v. United States, 361 U.S. 212, 218, 80 S.Ct. 270, 273, 4 L.Ed.2d 252 (1960); Hale v. Henkel, 201 U.S. 43, 61, 26 S.Ct. 370, 373, 50 L.Ed. 652 (1906); G. Edwards, The Grand Jury 28-32 (1906). Although the grand jury normally operates, of course, in the courthouse and under judicial auspices, its institutional relationship with the judicial branch has traditionally been, so to speak, at arm's length. Judges' direct involvement in the functioning of the grand jury has generally been confined to the constitutive one of calling the grand jurors together and administering their oaths of office. See United States v. Calandra, 414 U.S. 338, 343, 94 S.Ct. 613, 617, 38 L.Ed.2d 561 (1974); Fed.Rule Crim.Proc. 6(a).
"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 court, 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, 498 U.S. ----, ----, 111 S.Ct. 722, 726, 112 L.Ed.2d 795 (1991) (quoting United States v. Morton Salt Co., 338 U.S. 632, 642-643, 70 S.Ct. 357, 364, 94 L.Ed. 401 (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, 39 S.Ct. 468, 471, 63 L.Ed. 979 (1919). The grand jury requires no authorization from its constituting court to initiate an investigation, see Hale, supra, 201 U.S., at 59-60, 65, 26 S.Ct., at 373, 375, nor does the prosecutor require leave of court to seek a grand jury indictment. And in its day-to-day functioning, the grand jury generally operates without the interference of a presiding judge. See Calandra, supra, 414 U.S., at 343, 94 S.Ct., at 617. 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., at 424-425, 103 S.Ct., at 3138.
"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, 79 S.Ct. 539, 545, 3 L.Ed.2d 609 (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, 92 S.Ct. 2614, 33 L.Ed.2d 583 (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) (same with respect to privilege for confidential marital communications) (opinion of Kennedy, J.). Even in this setting, however, we have insisted that the grand jury remain "free to pursue its investigations unhindered by external influence or supervision so long as it does not trench upon the legitimate rights of any witness called before it." United States v. Dionisio, 410 U.S. 1, 17-18, 93 S.Ct. 764, 773, 35 L.Ed.2d 67 (1973). Recognizing this tradition of independence, we have said that the Fifth Amendment's "constitutional guarantee presupposes an investigative body 'acting independently of either prosecuting attorney or judge '. . . ." Id., at 16, 93 S.Ct., at 773 (emphasis added) (quoting Stirone, supra, 361 U.S., at 218, 80 S.Ct., at 273)."
