32 C.F.R. § 147.18
The following investigative standards are established for all United States Government civilian and military personnel, consultants, contractors, employees of contractors, licensees, certificate holders or grantees and their employees and other individuals who require access to classified information, to include Sensitive Compartmented Information and Special Access Programs, and are to be used by government departments and agencies as the investigative basis for final clearance determinations. However, nothing in these standards prohibits an agency from using any lawful investigative procedures in addition to these requirements in order to resolve any issue identified in the course of a background investigation or reinvestigation.
32 C.F.R. § 147.18