opinion: Justice Powell
  delivered the opinion of the Court.
  Pursuant to the federal use immunity provisions, 18 U. S. C. §§6001-6005, a United States Attorney may request an order from a federal court compelling a witness to testify even though he has asserted his privilege against self-incrimination. Section 6002 provides, however, that “no testimony or other information compelled under the order (or any information directly or indirectly derived from such testimony or other information) may be used against the witness in any criminal case . . . .” The issue presented in this case is whether a deponent’s civil deposition testimony, repeating verbatim or closely tracking his prior immunized testimony, is immunized “testimony” that can be compelled over the valid assertion of his Fifth Amendment privilege.
  t — I
  Respondent John Conboy is a former executive of a defendant in In re Corrugated Container Antitrust Litigation, M. D. L. 310 (SD Tex.). In January 1978, United States Department of Justice attorneys interviewed Conboy following a promise of use immunity. Conboy subsequently appeared before a grand jury investigating price-fixing activities and, pursuant to 18 U. S. C. § 6002, was granted formal use immunity for his testimony.
  Following the criminal indictment of several companies, numerous civil antitrust actions were filed in various United States District Courts. Those actions were consolidated for discovery in the District Court for the Southern District of Texas. Petitioners here are purchasers of corrugated containers who elected to opt out of the class-action proceedings and pursue their own causes of action against manufacturers. The District Court ordered that portions of the immunized Government interview and grand jury testimony of certain witnesses, including that of Conboy, be made available to lawyers for the class and opt-outs.
  Pursuant to a subpoena issued by the District Court for the Northern District of Illinois, Conboy appeared in Chicago for a deposition at which he, his counsel, and petitioners’ counsel had copies of his immunized testimony. The transcripts were marked as deposition exhibits so that all could follow the intended examination. The questioning fell into the following pattern: a question was read from the transcript; it then was rephrased to include the transcript answer (i. e., “Is it not the fact that . . finally, Conboy was asked if he had “so testified]” in his immunized interview and grand jury examination. Conboy refused to answer each question, asserting his Fifth Amendment privilege against self-incrimination.
  The District Court granted petitioners’ motion to compel Conboy to answer the questions. When Conboy continued to claim his privilege, the District Court held him in contempt, but stayed its order pending appeal. A panel of the Court of Appeals for the Seventh Circuit affirmed the contempt order, holding that, “[bjecause the questions asked in this deposition were taken verbatim from or closely tracked the transcript of Conboy’s grand jury testimony, we believe that his answers at the deposition would be ‘derived from’ the prior immunized [testimony] and therefore unavailable for use in any subsequent criminal prosecution.” In re Corrugated Container Antitrust Litigation, Appeal of Conboy, 655 F. 2d 748, 751 (1981).
  On rehearing en banc, the Court of Appeals reversed the District Court. 661 F. 2d 1145 (1981). It first determined that Conboy’s alleged fear of prosecution was more than “fanciful,” id., at 1152, and that Conboy therefore was entitled to assert his Fifth Amendment privilege unless his deposition testimony could not be used against him in a subsequent criminal action, see id., at 1153. The court then held that under § 6002, absent a separate and independent grant of immunity, a deponent’s civil deposition testimony that repeats verbatim or closely tracks his prior immunized testimony is not protected. While acknowledging that verbatim questions “of course [would be] derived” from the immunized testimony, the court reasoned that the answers to such questions “are derived from the deponent’s current, independent memory of events” and thus “necessarily create a new source of evidence” that could be used in a subsequent criminal prosecution against Conboy. Id., at 1155 (emphasis in original).
  We granted certiorari to resolve the conflict in the Courts of Appeals, 454 U. S. 1141 (1982), and now affirm.
  H h — I
  It is settled that government must have the power to compel testimony “to secure information necessary for effective law enforcement.” Murphy v. Waterfront Comm’n, 378 U. S. 52, 79 (1964). For many years, however, a person who was compelled to testify under a grant of governmental immunity could not be prosecuted for any conduct about which he had testified. See New Jersey v. Portash, 440 U. S. 450, 457 (1979). Prosecutors therefore were reluctant to grant such “transactional” immunity to potential targets of criminal investigations. See S. Rep. No. 91-617, p. 53 (1969).
  The “major purpose” of the Organized Crime Control Act of 1970, Pub. L. 91-452, 84 Stat. 922, of which § 6002 was a key provision, was “to provide the criminal justice system with the necessary legal tools to . . . strengthe[n] the evidence gathering process and insur[e] that the evidence will then be available and admissible at trial.” 116 Cong. Rec. 35200 (1970) (statement of Rep. St Germain). Congress sought to make the grant of immunity more useful for law enforcement officers through two specific changes. First, Congress made the grant of immunity less expansive by repealing the authority for transactional immunity and providing for the less comprehensive use immunity authorized in §6002. Second, Congress gave certain officials in the Department of Justice exclusive authority to grant immunities.
  The Court upheld the constitutionality of the use immunity statute in Kastigar v. United States, 406 U. S. 441 (1972). The power to compel testimony is limited by the Fifth Amendment, and we held that any grant of immunity must be coextensive with the privilege. We were satisfied, however, that §6002 provided this measure of protection and thus “removed the dangers against which the privilege protects.” Id., at 449. In rejecting the argument that use and derivative-use immunity would not adequately protect a witness from various incriminating uses of the compelled testimony, we emphasized that “[t]he statute provides a sweeping proscription of any use, direct or indirect, of the compelled testimony and any information derived therefrom . . . Id., at 460. We added that once a defendant establishes that he has testified under a grant of immunity, “the prosecution [has] the affirmative duty to prove that the evidence it proposes to use is derived from a legitimate source wholly independent of the compelled testimony.” Ibid. Thus, “immunity from use and derivative use ‘leaves the witness and the Federal Government in substantially the same position as if the witness had claimed his privilege’ in the absence of a grant of immunity.” Id., at 458-459 (quoting Murphy, 378 U. S., at 79).
  Ill
  With the foregoing statutory history and relevant principles in mind, we turn now to this case. It is not disputed that the questions asked of Conboy were directly or indirectly derived from his immunized testimony. The issue as presented to us is whether the causal connection between the questions and the answers is so direct that the answers also are derived from that testimony and therefore should be excluded under the grant of immunity.
  Petitioners’ argument is based on the language of §6002 and on a common understanding of the words “derived from.” The questions formulated on the basis of immunized testimony are clearly “derived from” the prior testimony. Thus, the answers that repeat verbatim or closely track a deponent’s testimony are necessarily also “derived from” and “tainted by” such testimony. Petitioners therefore find no basis for the distinction made by the Court of Appeals between questions and answers responsive to those same questions. An answer by its very nature is evoked by and responds to information contained in a question.
  Conboy’s position is also straightforward: Questions do not incriminate; answers do. Unlike the questions, answers are not directly or indirectly derived from the immunized grand jury or interview transcripts, but from the deponent’s current, independent memory of events. Even when a deponent’s deposition answers are identical to those he gave to the grand jury, he is under oath to tell the truth, not necessarily as he told it before the grand jury, but as he knows it now. Each new statement of the deponent creates a new “source.” In sum, the initial grant of immunity does not prevent the prosecutor from prosecuting; it merely limits his sources of evidence.
  Although the parties make their arguments in terms tracking those of the statute — whether the deposition testimony is “derived from” the prior testimony — it is clear that the crux of their dispute is whether the earlier grant of immunity itself compelled Conboy to talk. Petitioners contend that the prior grant of immunity already had supplanted Conboy’s Fifth Amendment privilege at the time of the civil deposition. Petitioners would limit this immunity, of course, to testimony that “closely tracks” his prior immunized testimony. It is argued that this would not threaten the Government’s need for admissible evidence or the individual’s interest in avoiding self-incrimination. In the absence of such a threat, admissible evidence should be available to civil antitrust plaintiffs. But we cannot accept the assumptions upon which petitioners’ conclusion rests. In our view, a District Court cannot compel Conboy to answer deposition questions over a valid assertion of his Fifth Amendment right, absent a duly-authorized assurance of immunity at the time.
  We note at the outset that although there may be practical reasons for not testifying, as far as the deponent’s Fifth Amendment right is concerned he should be indifferent between the protection afforded by silence and that afforded by immunity. A deponent’s primary interest is that the protection be certain. The Government’s interest, however, may be affected seriously by whether the deponent relies at the civil deposition on his Fifth Amendment privilege or on his prior grant of immunity. With due recognition of petitioners’ need for admissible evidence, our inquiry then is whether this need can be met without jeopardizing the Government’s interest in limiting the scope of an immunity grant or encroaching upon the deponent’s certainty of protection.
  A
  Questions taken verbatim from a transcript of immunized testimony could evoke one of several responses from a deponent: (i) he could repeat or adopt his immunized answer; (ii) he could affirm that the transcript of his immunized answers accurately reflects his prior testimony; (iii) he could recall additional information responsive to the question but not disclosed in his immune testimony; or (iv) he could disclose information that is not responsive to the question. Petitioners do not contend, nor could they, that the prior grant of use immunity affords protection for all self-incriminating information disclosed by the immunized witness on any occasion after the giving of the immunized testimony. Rather, petitioners argue that only the first three responses would be “derived from” his immune testimony and therefore would be unavailable for use against the deponent in any subsequent criminal prosecution.
  Petitioners’ premise is that the deposition of Conboy is designed not to discover new information, but to obtain evidence that simply repeats the statements in the immunized transcript. Because there will be little opportunity for the grant of immunity to sweep in statements on direct examination that the Government did not intend to immunize, or for the deponent to give responses that may fall outside of the grant of immunity and later be used against him in a subsequent criminal prosecution, petitioners argue that Conboy’s deposition will yield only a carbon copy of the grand jury transcript. In such a situation, it would be desirable for civil plaintiffs, particularly those bringing private suits that supplement the criminal enforcement of the federal antitrust laws, to have access to the available, probative information.
  But even if the direct examination is limited to the questions and answers in the immunized transcript, there remains the right of cross-examination, a right traditionally relied upon expansively to test credibility as well as to seek the truth. Petitioners recognize this problem, but maintain that the antitrust defendants “would be entitled to test the accuracy and truthfulness of Conboy’s repeated immunized testimony without going beyond the confines of that testimony.” Reply Brief for Petitioners 14-15. Regardless of any limitations that may be imposed on its scope, however, cross-examination is intended to and often will produce information not elicited on direct. We must assume that, to produce admissible evidence, the scope of cross-examination at the deposition cannot easily be limited to the immunized testimony. This assumption implicates both the Government’s and the individual’s interests embodied in § 6002.
  B
  Use immunity was intended to immunize and exclude from a subsequent criminal trial only that information to which the Government expressly has surrendered future use. If the Government is engaged in an ongoing investigation of the particular activity at issue, immunizing new information (e. g., the answers to questions in a case like this one) may make it more difficult to show in a subsequent prosecution that similar information was obtained from wholly independent sources. If a district court were to conclude in a subsequent civil proceeding that the prior immunity order extended to civil deposition testimony closely tracking the immunized testimony, it in effect could invest the deponent with transactional immunity on matters about which he testified at the immunized proceedings. This is precisely the kind of immunity Congress intended to prohibit. The purpose of § 6002 was to limit the scope of immunity to the level that is constitutionally required, as well as to limit the use of immunity to those cases in which the Attorney General, or officials designated by him, determine that gaining the witness’ testimony outweighs the loss of the opportunity for criminal prosecution of that witness.
  C
  Petitioners’ interpretation of § 6002 also places substantial risks on the deponent. Unless the grant of immunity assures a witness that his incriminating testimony will not be used against him in a subsequent criminal prosecution, the witness has not received the certain protection of his Fifth Amendment privilege that he has been forced to exchange. No court has authority to immunize a witness. That responsibility, as we have noted, is peculiarly an executive one, and only the Attorney General or a designated officer of the Department of Justice has authority to grant use immunity. See 18 U. S. C. §§6002, 6003. Nor should a court, at the time of the civil testimony, predetermine the decision of the court in a subsequent criminal prosecution on the question whether the Government has met its burden of proving that “the evidence it proposes to use is derived from a legitimate source wholly independent of the compelled testimony.” Kastigar, 406 U. S., at 460. Yet in holding Conboy in contempt for his Fifth Amendment silence, the District Court below essentially predicted that a court in any future criminal prosecution of Conboy will be obligated to protect against evidentiary use of the deposition testimony petitioners seek. We do not think such a predictive judgment is enough.
  Petitioners’ interpretation of §6002 imposes risks on the deponent whether or not the deposition testimony properly can be used against him in a subsequent criminal prosecution. Accordingly, the District Court’s compulsion order in this case, in the absence of statutory authority or a new grant of immunity by the United States Attorney, cannot be justified by the subsequent exclusion of the compelled testimony. As Justice Marshall notes in his concurring opinion: “Whatever justification there may be for requiring a witness to give incriminating testimony in aid of a criminal investigation after the Government has granted use immunity, there is no similar justification for compelling a witness to give incriminating testimony for the benefit of a private litigant when the Government has not chosen to grant immunity.” Post, at 267.
  The result of compelling testimony — whether it is immunized or excluded — is that the Government’s interests, as well as the witness’, suffer. Reliance on judicial exclusion of nonimmunized testimony would be inconsistent with the congressional policy of leaving the granting of immunity to the Executive Branch.
  As the Court stated in Maness v. Meyers, 419 U. S. 449 (1975), compelling a witness to testify in “reliance upon a later objection or motion to suppress would ‘let the cat out’ with no assurance whatever of putting it back.” Id., at 463. We believe Conboy acted properly in maintaining his silence in the face of the District Court’s compulsion order and by testing the validity of his privilege on appeal.
  >
  This Court has emphasized the importance of the private action as a means of furthering the policy goals of certain federal regulatory statutes, including the federal antitrust laws. See, e. g., Perma Life Mufflers, Inc. v. International Parts Corp., 392 U. S. 134, 139 (1968); United States v. Borden Co., 347 U. S. 514, 518-519 (1954). But private civil actions can only supplement, not supplant, the primary responsibility of government. Petitioners’ proposed construction of § 6002 sweeps further than Congress intended and could hinder governmental enforcement of its criminal laws by turning use immunity into a form of transactional immunity for subjects examined in the immunized proceeding. It also puts the deponent in some danger of criminal prosecution unless he receives an assurance of immunity or exclusion that the courts cannot properly give. Silence, on the other hand, preserves the deponent’s rights and the Government’s interests, as well as the judicial resources that otherwise would be required to make the many difficult judgments that petitioners’ interpretation of § 6002 would require.
  V
  We hold that a deponent’s civil deposition testimony, closely tracking his prior immunized testimony, is not, without duly authorized assurance of immunity at the time, immunized testimony within the meaning of § 6002, and therefore may not be compelled over a valid assertion of his Fifth Amendment privilege. The judgment of the Court of Appeals accordingly is
  Affirmed.
  The propriety of the District Court's release of grand jury materials to the civil parties is not before the Court.
  An example of this three-question pattern is as follows:
  “Q. Who did you have price communications with at Alton Box Board?
  “Q. Is it not the fact that you had price communications with Fred Renshaw and Dick Herman . . . ?
  “Q. Did you not so testify in your government interview statement of January 10, 1978?” App. 29-31.
  Chief Judge John V. Singleton, Jr., of the District Court for the Southern District of Texas expressly exercised the powers of the District Court for the Northern District of Illinois pursuant to 28 U. S. C. § 1407(b). The contempt hearing was conducted by telephone with his chambers in Houston.
  The correctness of the Court of Appeals’ conclusion that Conboy could assert a Fifth Amendment privilege, absent some immunity, is not before us.
  A United States Attorney declined to authorize immunity grants in connection with the civil depositions here.
  Compare In re Corrugated, Container Antitrust Litigation, Appeal of Fleischacker, 644 F. 2d 70, 75 (CA2 1981) (deposition answers immunized), and Little Rock School District v. Borden, Inc., 632 F. 2d 700, 705 (CA8 1980) (same), with In re Corrugated Container Anti-Trust Litigation, Appeal of Franey, 620 F. 2d 1086,1095 (CA5 1980) (answers not immunized), cert. denied, 449 U. S. 1102 (1981).
  See United States v. Calandra, 414 U. S. 338, 345 (1974); United States v. Mara, 410 U. S. 19, 41 (1973) (Marshall, J., dissenting); Kastigar v. United States, 406 U. S. 441, 443-444 (1972); Murphy, 378 U. S., at 93-94 (White, J., concurring); Blackmer v. United States, 284 U. S. 421, 438 (1932); Blair v. United States, 250 U. S. 273, 281 (1919); Brown v. Walker, 161 U. S. 591, 600 (1896).
  In Murphy, Justice White stated that “[(Immunity must be as broad as, but not harmfully and wastefully broader than, the privilege against self-incrimination.” 378 U. S., at 107 (concurring opinion) (quoted with approval in 116 Cong. Rec. 35291 (1970) (statement of Rep. Poff)). In its Committee Report, the House explained that § 6002 was not to provide an “immunity bath,” but was to be “no broader than” the Fifth Amendment privilege. H. R. Rep. No. 91-1549, p. 42 (1970).
  Section 6002 provides:
  “Whenever a witness refuses, on the basis of his privilege against self-incrimination, to testify or provide other information in a proceeding before or ancillary to—
  “(1) a court or grand jury of the United States,
  “(2) an agency of the United States, or
  “(3) either House of Congress, a joint committee of the two Houses, or a committee or a subcommittee of either House
  and the person presiding over the proceeding communicates to the witness an order issued under this part, the witness may not refuse to comply with the order on the basis of his privilege against self-incrimination; but no testimony or other information compelled under the order (or any information directly or indirectly derived from such testimony or other information) may be used against the witness in any criminal case, except a prosecution for perjury, giving a false statement, or otherwise failing to comply with the order.”
  Section 6001(2) defines "other information” to include “any book, paper, document, record, recording, or other material.”
  Section 6003 states:
  “(a) In the case of any individual who has been or may be called to testify or provide other information at any proceeding before or ancillary to a court of the United States or a grand jury of the United States, the United States district court for the judicial district in which the proceeding is or may be held shall issue, in accordance with subsection (b) of this section, upon the request of the United States attorney for such district, an order requiring such individual to give testimony or provide other information which he refuses to give or provide on the basis of his privilege against self-incrimination, such order to become effective as provided in section 6002 of this part.
  “(b) A United States attorney may, with the approval of the Attorney General, the Deputy Attorney General, or any designated Assistant Attorney General, request an order under subsection (a) of this section when in his judgment—
  “(1) the testimony or other information from such individual may be necessary to the public interest; and
  “(2) such individual has refused or is likely to refuse to testify or provide other information on the basis of his privilege against self-incrimination.”
  Congress foresaw the courts as playing only a minor role in the immunizing process: “The court’s role in granting the order is merely to find the facts on which the order is predicated.” H. R. Rep. No. 91-1549, supra, at 43; H. R. Rep. No. 91-1188, p. 13 (1970). See 116 Cong. Rec. 35291 (1970) (statement of Rep. Poff). Cf. President’s Commission on Law Enforcement and Administration of Justice, The Challenge of Crime in a Free Society 141 (1967) (recommending that “[(Immunity should be granted only with the prior approval of the jurisdiction’s chief prosecuting officer”).
  See Brief for Petitioners 9 (“Conboy had no Fifth Amendment privilege to assert because of the coextensive protection provided by the immunity statute”); Reply Brief for Petitioners 12 (“[R]equiring a witness to answer questions a second time that were previously answered under a grant of immunity does not result in an expansion of the original immunity grant”).
  Justice Blackmun, concurring in the Court’s judgment, assumes that Conboy had a right to remain silent at the deposition, which by definition assumes the immunity order itself does not compel a witness to testify at a civil deposition. He discusses the “fruits” doctrine where a witness’ testimony at a deposition is “an independent act of free will” and concludes that “had Conboy answered the deposition questions, his testimony would not have been protected by the original immunity grant. .. . .” Post, at 280. We have no occasion to address this hypothetical. The issue is whether Conboy can be compelled to testify — i. e., whether the immunity order compels him to track his prior testimony at the civil deposition — over the assertion of his Fifth Amendment rights. If, as we conclude, the original grant of immunity does not extend to the subsequent civil proceeding, then the trial judge lacks authority to compel Conboy to testify over the assertion of his Fifth Amendment privilege. This is so irrespective of whether, had he testified at the deposition rather than asserting the privilege, his answers could have been admitted against him at a criminal trial. We therefore need not now decide the extent to which civil deposition testimony, freely given by a witness in Conboy’s position, is “directly or indirectly derived” from prior grand jury testimony.
  As Justice Blackmun’s opinion makes a factual analysis under the “fruits” doctrine, it appears to leave open the possibility that the outcome in a subsequent criminal prosecution of the deponent may be different in a future case because of differences in the factual record. He nevertheless concludes, as do we, that district courts are without power to compel a civil deponent to testify over a valid assertion of his Fifth Amendment right, absent a separate grant of immunity pursuant to § 6002.
  Besides the costs of testifying against close associates, any witness increases the risk of committing perjury the more he talks. Cf. 18 U. S. C. § 6002 (perjured testimony not immunized).
  The extreme case would be where petitioners read the entire immunized grand jury transcript; then ask the witness if that is his testimony; and he answers simply “Yes.”
  Direct examination may not be as limited as petitioners assume. The District Court’s civil contempt order stated that the questions asked in the deposition “were taken directly” from the immunized transcripts, but did not define exactly what deposition questions petitioners could ask. Other Courts of Appeals have permitted direct questioning to go beyond mere restatements of the prior testimony. See In re Corrugated Container Antitrust Litigation, Appeal of Fleischacker, 644 F. 2d, at 79 (compelling answers to questions “concerning specific subjects that actually were touched upon by questions appearing in the transcript of the immunized testimony”); Little Rock School District v. Borden, Inc., 632 F. 2d, at 705 (compelling answers as long as deposition questions confined to “ ‘the same time, geographical and substantive frame work as the [witness’ immunized] grand jury testimony’”) (quoting Appeal of Starkey, 600 F. 2d 1043, 1048 (CA8 1979)). The dissenting opinion of Justice Stevens apparently does not attempt to indicate when questioning will exceed proper limits.
  For purposes of this case, we assume that the grand jury transcripts are inadmissible as evidence in a civil trial because the testimony is not subject to cross-examination. Cf. Fed. Rule Evid. 803(8) (hearsay exception for certain public records); Fed. Rule Evid. 804(a)(1) (witness unavailable when exempted from testifying on ground of privilege); Fed. Rule Evid. 804(b)(1) (former testimony admissible when witness unavailable and the party against whom the testimony is now offered had an opportunity for cross-examination).
  Cf. Fed. Rule Civ. Proc. 26(b)(1) (stating that depositions may be taken “if the information sought appears reasonably calculated to lead to the discovery of admissible evidence”); Fed. Rule Civ. Proc. 30(c) (allowing cross-examination at depositions); Fed. Rule Civ. Proc. 32(a) (deposition “admissible under the rules of evidence applied as though the witness were then present and testifying”); Fed. Rule Evid. 804(b)(1) (deposition admissible if the party against whom the testimony is now offered in a civil action had an opportunity to develop testimony by cross-examination).
  See United States v. Cardillo, 316 F. 2d 606, 611 (CA2 1963) (in determining whether testimony of a witness who invokes the privilege during cross-examination may be used against defendant, court draws a distinction between cases in which the assertion of the privilege merely precludes inquiry into collateral matters that bear on credibility of witnesses and those in which assertion prevents inquiry into matters about which witness testified on direct).
  We need not decide whether United States Attorneys, when designated by the Attorney General, presently have authority to immunize the testimony of a witness in a civil proceeding when the Government determines that the public interest would be served.
  None of the tests set forth by Courts of Appeals that have adopted petitioners’ interpretation of § 6002 provides deponents with certain guidance as to when they must talk and when they must not. See n. 16, supra.
  Cf. post, at 268 (MARSHALL, J., concurring) (“Further incriminating evidence that is derived from compelled testimony cannot always be traced back to its source”); n. 14, supra (increasing risk of harm and perjury); n. 23, infra (increasing exposure to civil liability).
  The dissent minimizes the enforcement interest that our construction of § 6002 protects, post, at 288-290, contending that we “misunderstood] the prosecutorial interest,” post, at 288. We note, however, that by conceding that there is some “risk” that the deponent’s testimony may hamper a prosecution, post, at 293, the dissent concedes that its interpretation of § 6002 provides at least somewhat broader immunity than Congress intended. Moreover, the dissent overlooks the possible difficulty of securing the cooperation of individuals such as Conboy who may be more reluctant to testify in the immunized proceedings if they know that later deposition testimony may increase their exposure to civil liability. Finally, in the dissent’s judgment, “the theoretical risk that compelled testimony could hamper a potential prosecution [is] plainly outweighed by the enforcement interest in allowing the deposition to go forward.” Ibid. See also post, at 289. This, however, is a judgment reserved for officials of the Department of Justice, not the federal courts, to make on a case-by-case basis.
  Our holding is limited to precluding district courts from compelling testimony in a civil deposition over a valid assertion of the Fifth Amendment privilege, absent a specific assurance of immunity for such testimony.