Statutory Exceptions to Posse Comitatus Act

"DOD [Department of Defense] personnel are permitted to provide training and expert advice to civilian law enforcement personnel, and may conduct maintenance on equipment it provides. However, DOD personnel are expressly authorized to operate the DOD-provided equipment only in support of certain federal law enforcement operations, which include counter-terrorism operations, renditions of suspected terrorists from a foreign country to the United States to stand trial, and investigations involving violations of certain laws that control imports, exports, immigration, drug trafficking, and terrorism.87 DOD personnel are authorized to operate equipment for the purpose of, among other things, detection, monitoring, and communication of the movement of air and sea traffic, as well as surface traffic outside of the geographic boundary of the United States and within the United States not to exceed 25 miles of the boundary (if the initial detection occurred outside of the boundary), and “aerial reconnaissance.”"

Source: 

Best, Richard A., Jr.; Elsea, Jennifer K., "Satellite Surveillance: Domestic Issues," Congressional Research Service (Washington, DC: Library of Congress, January 13, 2011), p. 20.
http://www.fas.org/sgp/crs/int...

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