29 C.F.R. § 1410.12
With regard to Agency Internal Personnel Records and Arbitrator Personal Data Files, separately described in the system notices, such records will be exempted from section (d) of the Act as follows:
Investigatory material maintained solely for the purposes of determining an individual's qualification, eligibility, or suitability for employment in the Federal civilian service, Federal contracts, or access to classified information, but only to the extent that disclosure of such material would reveal the identity of the source who furnished information to the Government under an express promise that the identity of the source would be held in confidence, or prior to September 27, 1975, under an implied promise that the identity of the source would be held in confidence.
In order to obtain accurate information pertaining to employee or arbitrator eligibility, the nondisclosure of the identity of such a confidential source is essential.
29 C.F.R. § 1410.12