Military Surveillance of US Citizens

According to US Defense Department regulation 5240.1-R, DoD intelligence components are allowed to gather information on US citizens in some circumstances relating to drugs: "C2.3.10. Narcotics. Information may be collected about a United States person who is reasonably believed to be engaged in international narcotics activities."

Source: 

Under Secretary of Defense for Policy, "Procedures Governing the Activities of DOD Intelligence Components That Affect United States Persons," (Washington, DC: United States Department of Defense, December 1982), p. 17.
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