5 C.F.R. § 5.4
When required by the Office, the Merit Systems Protection Board, or the Special Counsel of the Merit Systems Protection Board, or by authorized representatives of these bodies, agencies shall make available to them, or to their authorized representatives, employees to testify in regard to matters inquired of under the civil service laws, rules, and regulations, and records pertinent to these matters. All such employees, and all applicants or eligibles for positions covered by these rules, shall give to the Office, the Merit Systems Protection Board, the Special Counsel, or to their authorized representatives, all information, testimony, documents, and material in regard to the above matters, the disclosure of which is not otherwise prohibited by law or regulation. These employees, applicants, and eligibles shall sign testimony given under oath or affirmation before an officer authorized by law to administer oaths. Employees are performing official duty when testifying or providing evidence pursuant to this section.
5 C.F.R. § 5.4