U.S. Code Title 5 Section (b)(6)

Section 552. Public information; agency rules, opinions, orders, records, and proceedings
(b) This section does not apply to matters that are -
(6) personnel and medical files and similar files the
disclosure of which would constitute a clearly unwarranted
invasion of personal privacy;

This means that one or more of the subjects of the investigation are Government employees and they are alleging that releasing this report would violate their "privacy".