"Before an employee of the Department of Defense is allowed access to classified information, a background investigation is conducted by the Defense Investigative Service (DIS). The purpose of the investigation is to determine whether issues related to loyalty, reliability, and trustworthiness are present in the person's background. Two major background areas of concern are general suitability issues and loyalty issues. Suitability issues are alcohol use, drug use, criminal conduct, financial responsibility, emotional/mental issues, sexual misconduct and falsification. Loyalty issues are allegiance to the United States, foreign connection, and foreign preferences. A third area, security incidents, is concerned with responsibility for handling classified information. The DIS conducts investigations on three populations of DOD employees: military, both active and reserve; Federal civilians; and defense contractors. Between 1987 and 1996, DIS performed quality assurance by reviewing a sample of issue cases."
Defense Technical Information Center (DTIC): http://www.dtic.mil/dtic/