Plaintiffs Kenneth and Madeline Barney appeal the district court's1 entry of summary judgment against them and dismissal of their complaint brought under the Freedom of Information Act (FOIA), 5 U.S.C. § 552, to obtain information from the Internal Revenue Service (IRS). For the reasons stated below, we affirm but remand with direction.

The Barneys are currently under investigation by the IRS due to their alleged failure to file federal income tax returns for 1975. After unsuccessful attempts to obtain IRS records of interest to them, plaintiffs initiated this action pursuant to the FOIA in district court, seeking an order requiring the IRS to disclose to them seventeen categories of information.

The IRS responded with a motion for summary judgment and supporting affidavits and exhibits. The IRS advised that it was willing to release to plaintiffs 952 pages out of a total of 1,975 pages of documents in issue.2 The IRS further advised that it had no records reflecting the information requested in plaintiffs' categories 4, 6, 7, 8, 12, 13, and 17,3 and that it could not identify from plaintiffs' description the records requested in category 9.4 Finally, the IRS objected to release of the documents requested in categories 1, 2, 3 and 5,5 contending that all such information is exempt from disclosure under the FOIA, specifically by exemption 7(A)6 and by exemption 3 in conjunction with 26 U.S.C. § 6103(e)(6).7

After reviewing the government's motion and supporting affidavits, the district court entered an order granting the motion for summary judgment, ruling that all the disputed records are exempt from disclosure. Specifically, the court concluded

(1) that all of the documents and records at issue are exempt from disclosure under 5 U.S.C. § 552(b)(7)(A) in that these records represent investigatory records compiled for law enforcement purposes and that production thereof of any of the documents and records at this time would result in an interference with a law enforcement proceeding; and

(2) that all of the documents and records at issue are exempt from disclosure under 5 U.S.C. § 552(b)(3), which incorporates Section 6103(e)(6) of the Internal Revenue Code of 1954, as amended, in that production at this time of the documents covered by plaintiffs' Freedom of Information Act request would seriously impair Federal tax administration.

The Barneys timely appealed from this order and from a subsequent order reinstating the summary judgment.8

On appeal the Barneys raise numerous objections but only one warrants any extended discussion. Plaintiffs object to the procedures utilized by the district court in making the determination that all the withheld documents fall within exemptions 7(A) and 3 of the FOIA. The government affidavits describe the withheld documents in fairly general terms.9 Nevertheless, the district court found the affidavits adequate to permit the determination that all disputed documents are exempt from disclosure. Plaintiffs contend that the district court could not make a proper de novo determination without in camera inspection of the documents or requiring the government to submit a detailed index, itemizing the documents withheld and specifying the applicable exemptions.

5 U.S.C. § 552(a)(4)(B) provides that upon complaint, the district court is to make a de novo determination of whether records were properly withheld under any of the FOIA exemption provisions. The burden is on the agency to sustain its action. Id. In making its determination, the district court "may examine the contents of such agency records in camera." Id. (emphasis added). It is clear that in camera inspection is not mandated by the Act, but lies within the discretion of the district court. NLRB v. Robbins Tire & Rubber Co., 437 U.S. 214, 224, 98 S. Ct. 2311, 2317, 57 L. Ed. 2d 159 (1978).

We have previously indicated that the role of in camera inspection should be limited as it is "contrary to the traditional judicial role of deciding issues in an adversarial context upon evidence openly produced in court." Cox v. United States Department of Justice, 576 F.2d 1302, 1311 (8th Cir. 1978).

A court's primary role * * * is to review the adequacy of the affidavits and other evidence presented by the Government in support of its position * * *. If the Government fairly describes the content of the material withheld and adequately states its grounds for nondisclosure, and if those grounds are reasonable and consistent with the applicable law, the district court should uphold the Government's position. The court is entitled to accept the credibility of the affidavits, so long as it has no reason to question the good faith of the agency.

Id. at 1312.

Plaintiffs' contention that the government is required to submit a detailed index of the withheld documents derives from opinions of the D.C. Circuit. In Vaughn v. Rosen, 484 F.2d 820 (D.C.Cir.1973), cert. denied, 415 U.S. 977, 94 S. Ct. 1564, 39 L. Ed. 2d 873 (1974), the D.C. Circuit established that "an agency's response to a FOIA request must include an index of all material withheld in whole or in part." Founding Church of Scientology, Inc. v. Bell, 603 F.2d 945, 947 (D.C.Cir.1979). The index "must adequately describe each withheld document or deletion from a released document," and "must state the exemption claimed for each * * * and explain why the exemption is relevant." Id. at 949. This indexing procedure is perceived as necessary to permit the district court and the requesting party to evaluate the agency's decision to withhold records and to ensure its compliance with the mandates of the FOIA. Id. at 947.

While we acknowledge that in some cases a Vaughn index or in camera inspection may be necessary to permit a proper de novo determination of exemption, this is not such a case. To sustain its burden of showing documents were properly withheld under exemption 7(A) the government had to establish only that they were investigatory records compiled for law enforcement purposes and that production would interfere with pending enforcement proceedings. The affidavits of Special Agent Charles Smith and Patrick Sweetman, Acting District Director, Aberdeen District IRS, establish that the Barneys are currently under investigation by the IRS and may be subject to criminal prosecution.10 The Smith affidavit describes the withheld documents in sufficient detail to permit the determination that they were compiled in the course of this investigation and for law enforcement purposes.11 Moreover, this is apparent by the very nature of plaintiffs' request. In effect, they request disclosure of all investigatory records compiled on them by the IRS.12

The affidavits of Smith and Sweetman also adequately demonstrate that release of the documents would "interfere with enforcement proceedings" within the meaning of exemption 7(A).13 Under exemption 7(A) the government is not required to make a specific factual showing with respect to each withheld document that disclosure would actually interfere with a particular enforcement proceeding. NLRB v. Robbins Tire & Rubber Co., supra, 437 U.S. at 234-35, 98 S. Ct. at 2323. Rather, federal courts may make generic determinations that, "with respect to particular kinds of enforcement proceedings, disclosure of particular kinds of investigatory records while a case is pending would generally 'interfere with enforcement proceedings.' " Id. at 236, 98 S.Ct. at 2324. This is clearly the kind of case where such a generic determination is appropriate.

One of the the primary purposes of exemption 7 was "to prevent 'harm (to) the Government's case in court' * * * by not allowing litigants 'earlier or greater access' to agency investigatory files than they would otherwise have * * *." Id. at 224-25, 98 S.Ct. at 2318. We consider that disclosure of such records as witness statements, documentary evidence, agent's work papers and internal agency memoranda, prior to the institution of civil or criminal tax enforcement proceedings, would necessarily interfere with such proceedings by prematurely revealing the government's case. See Stephenson v. IRS, 79-2 U.S.T.C. 9513, 44 AFTR2d 79-5282 (D.Ga.1979). "FOIA was not intended to function as a private discovery tool * * *." NLRB v. Robbins Tire & Rubber Co., supra, 437 U.S. at 242, 98 S. Ct. at 2327. If and when enforcement proceedings are instituted the Barneys will be able to obtain access to such of this information as is discoverable through the normal pre-trial discovery channels. Moreover, once enforcement proceedings are either concluded or abandoned, exemption 7(A) will no longer apply to prevent disclosure, id. at 235, 98 S.Ct. at 2323, and the Barneys may seek release of the records under the FOIA at that time.

In sum, the government in this case "fairly describe(d) the content of the material withheld and adequately state(d) its grounds for nondisclosure, and * * * those grounds are reasonable and consistent with the applicable law." Cox v. United States Department of Justice, supra, 576 F.2d at 1312. Since plaintiffs offered no substantial reason to call into question to good faith of the agency,14 the district court was entitled to rely on the credibility of the affidavits. Id. Accordingly, the district court properly upheld the agency's determination of exemption without in camera inspection of the documents and without requiring the government to prepare a detailed index. We reiterate that in another case involving different exemption provisions a more comprehensive showing on the part of the government may be required.15

The remaining allegations of error require only cursory discussion. Plaintiffs complain that the district court failed to rule on several of their motions. However, these were all effectively disposed of by the grant of summary judgment. Plaintiffs apparently contend the government should be required to itemize the documents it intends to disclose to plaintiffs. Since plaintiffs will be able to examine the documents themselves, this claim is frivolous and their motion for itemization was property denied by the district court. Plaintiffs also contend the government should be required to disclose the names and titles of the persons responsible for withholding the documents. The government responds that the names and titles of the responsible persons were identified in the letters from the IRS denying plaintiffs' request, which letters plaintiffs attached as exhibits to their complaint. Since the government so identified the responsible persons, the court properly denied plaintiffs' motion for disclosure. Finally, plaintiffs attack the constitutionality of the income tax as applied to individuals and claim the IRS is therefore without authority to investigate them. This claim is frivolous. Attacks on the constitutionality of the income tax have been repeatedly rejected by this court. See Reading v. Commissioner, 614 F.2d 159 (8th Cir. 1980).

Affirmed in all respects except that this cause is remanded with direction that further agency action be taken with respect to category 9 discussed in note 4 of this opinion.