Activities in Violation of Posse Comitatus Act
"Questions regarding which activities violate the Posse Comitatus Act arise most often in the context of assistance to civilian police. At least in that context, the courts have held that, absent a recognized exception, the act is violated (1) when civilian law enforcement officials make 'direct active use' of military investigators, (2) when the use of the military 'pervades the activities' of the civilian officials, or (3) when the military is used so as to subject citizens to the exercise of military power that is 'regulatory, prescriptive, or compulsory in nature.'84 The act does not apply to the Navy or Marines85 and does not prohibit activities conducted for a military purpose that incidentally benefit civilian law enforcement bodies."
Best, Richard A., Jr.; Elsea, Jennifer K., "Satellite Surveillance: Domestic Issues," Congressional Research Service (Washington, DC: Library of Congress, January 13, 2011), p. 19.