Plaintiff-appellant (Lewis) appeals the district court’s order granting summary judgment for the defendant-appellee (IRS). Lewis brought this action under the Freedom of Information Act (FOIA), 5 U.S.C. § 552. Lewis sought an order directing the IRS to produce for his inspection and copying certain documents related to a criminal investigation of his failure to file tax returns for the years 1981 through 1983. In the alternative, Lewis sought an order directing the IRS to provide him an index of any withheld documents. The district court granted the IRS’s motion for summary judgment. The district court had jurisdiction pursuant to 5 U.S.C. § 552(a)(4)(B). Lewis’ appeal was timely. We have jurisdiction pursuant to 28 U.S.C. § 1291, and we affirm.
I.
In July and August of 1984, Lewis mailed to the District Director of the IRS in Anchorage, Alaska two requests for copies of various documents related to an ongoing criminal investigation of Lewis for violations of 26 U.S.C. § 7203 (willful failure to file a tax return) for the years 1981, 1982, and 1983. 1 The IRS sent Lewis copies of most of the requested documents but withheld approximately 700 pages, explaining to Lewis that because those pages were relevant to an ongoing criminal investigation they were considered exempt from disclosure under two provisions of FOIA. 2
Lewis wrote to the Commissioner of the IRS appealing the partial denial of his requests. The Commissioner did not respond, and Lewis then filed this action against the IRS alleging improper withholding of the requested documents. Additionally, Lewis moved for an index of the withheld documents pursuant to
Vaughn v. Rosen,
II.
We use a two-step analysis to review FOIA claims.
Dirksen v. United States Dep’t of Health & Human Serv.,
III.
Lewis contends that the district court did not have an adequate factual basis on which to decide whether the requested documents were properly withheld. He argues that the district court therefore should have conducted an in camera inspection of the documents rather than rely on affidavits presented by the IRS. We disagree.
While the IRS has the burden of establishing that the requested documents are exempt from disclosure, 5 U.S.C. § 552(a)(4)(B);
Scientology,
But if the affidavits are “too generalized,” the district court may,
in its discretion,
examine the disputed documents in camera in order to make “a first-hand determination of their exempt status.”
Scientology,
The affidavits submitted in this case described in sufficient detail the undisclosed materials. In addition, they adequately addressed why disclosure would impair the IRS’s investigation of Lewis for criminal tax violations. It is undisputed that release of the requested 700 pages of the documents would reveal, among other things, the limits and scope of the IRS’s case against Lewis, the names of third parties whom the IRS had contacted as well as the names of actual and potential witnesses. Furthermore, disclosure might enable Lewis to tamper with evidence which the IRS might subsequently request. 5 As we noted above, Lewis himself *379 specifically requested materials which relate to the ongoing criminal investigation. 6
The district court therefore had an adequate factual basis to make its decision that the documents were exempt from disclosure.
Cf. Harvey’s Wagon Wheel,
IV.
Assuming that the district court had an adequate factual basis, Lewis contends that the district court’s conclusion that the 700 pages which the IRS withheld were exempt from disclosure and that Lewis was therefore not entitled to a Vaughn index is clearly erroneous. The district court found that the documents which Lewis sought fell within two exemptions to FOIA, 5 U.S.C. §§ 552(b)(3)(B) (Exemption 3) and 552(b)(7)(A) (Exemption 7(A)). The district court therefore denied Lewis’ request for disclosure and for a Vaughn index. We find no clear .error in the district court’s ruling that Exemption 7(A) applies. Therefore, we affirm the judgment of the district court denying Lewis a Vaughn index.
The applicable portion of Exemption 7(A) exempts from disclosure “investigatory records compiled for law enforcement purposes, but only to the extent that the production of such records would ... interfere with enforcement proceedings.” 29 U.S.C. § 552(b)(7)(A). In order to sustain its burden of proof, the IRS must establish that it is a law enforcement agency, that the withheld documents were investigatory records compiled for law enforcement purposes, and that disclosure of those documents would interfere with pending enforcement proceedings.
See FBI v. Abramson,
The IRS has the requisite law enforcement mandate and the affidavits presented in this case establish a rational nexus between enforcement of a federal law and the documents for which the exemption is claimed.
Cf. Binion v. United States Dep’t of Justice,
Because the affidavits demonstrate that the IRS’s purpose in withholding the documents fell within the sphere of its law enforcement authority and that their release could interfere with their enforcement procedures, withholding of the documents under Exemption 7(A) was proper. The district court, reviewing the question de novo, agreed with the IRS that Exemption 7(A) applied, that the IRS has demonstrated that the documents relate to an ongoing criminal investigation, and that their premature release “would jeopordize future enforcement proceedings.”
NLRB v. Robbins Tire & Rubber Co.,
Finally, we note that the purpose of a
Vaughn
index is to aid a district court in its ruling on claimed exemptions to FOIA.
Vaughn,
AFFIRMED. 9
Notes
. The relevant portion of Lewis’ first letter requested “copies of any and all documents, interoffice memos, papers, or reports which bear [his] name, and are relative to the criminal investigation which the I.R.S. has commenced concerning [him].” (Emphasis added.) The relevant portion of Lewis’ second letter requested all documents which related to him and were "maintained in [the IRS’s] system of records known as ’Controlled Accounts, open and closed, Criminal Investigation Division.’ ” (Emphasis added.)
. 5 U.S.C. §§ 552(b)(3)(B) (Exemption 3) and 552(b)(7)(A) (Exemption 7(A)). Exemption 3 authorizes the nondisclosure of any documents which are "specifically exempted from disclosure by statute ... provided that such statute (A) requires that the matters be withheld from the public in such a manner as to leave no discretion on the issue, or (B) establishes particular criteria for withholding or refers to particular types of matters to be withheld." 29 U.S.C. § 552(b)(3).
See generally Julian v. United States Dep't of Justice,
.A
Vaughn
index is a system of itemizing and indexing that correlates each of the government’s justifications for its refusal to disclose the documents with the actual portions of the documents at issue.
See
. “Before the court orders in camera inspection, the Government should be given the opportunity to establish by means of testimony or detailed affidavits that the documents are clearly exempt from disclosure. The burden remains on the Government under this law.” S.Rep. No. 1200, 93d Cong., 2d Sess. 9, reprinted in 1974 U.S.Code Cong. & Ad.News 6267, 6287-88.
. According to one of the affidavits:
Disclosure of the documents ... would interfere with the current Service investigation of the plaintiff by prematurely revealing the evidence developed against the plaintiff; the reliance placed by the government on that evidence; the names of witnesses and potential witnesses; the scope and limits of the investigation; the identities of third parties contacted; the specific transactions being investigated; the strengths and weaknesses of the government’s case; and potential impeachment material. In addition, disclosure could aid plaintiff in tampering with potential evidence and witnesses, or otherwise frustrating the government’s ability to present its best case in court.
Lewis does not rebut this statement nor does he point to any bad faith on the part of the affiant.
. See note 1 supra.
. See note 1 supra.
. See note 5 supra.
. The IRS maintains that the withheld documents were also exempt under Exemption 3 in conjunction with 26 U.S.C. § 6103(e)(7).
Cf. Long v. United States IRS,
