And from Northcom:
http://www.northcom.mil/index.cfm?fuseacti...ets&factsheet=5
The United States Congress has enacted a number of exceptions to the PCA that allow the military, in certain situations, to assist civilian law enforcement agencies in enforcing the laws of the U.S. The most common example is counterdrug assistance (Title 10 USC, Sections 371-381). Other examples include:
The Insurrection Act (Title 10 USC, Sections 331-335). This act allows the president to use U.S. military personnel at the request of a state legislature or governor to suppress insurrections. It also allows the president to use federal troops to enforce federal laws when rebellion against the authority of the U.S. makes it impracticable to enforce the laws of the U.S.
Assistance in the case of crimes involving nuclear materials (Title 18 USC, Section 831). This statute permits DoD personnel to assist the Justice Department in enforcing prohibitions regarding nuclear materials, when the attorney general and the secretary of defense jointly determine that an “emergency situation” exists that poses a serious threat to U.S. interests and is beyond the capability of civilian law enforcement agencies.
Emergency situations involving chemical or biological weapons of mass destruction (Title 10 USC, Section 382). When the attorney general and the secretary of defense jointly determine that an “emergency situation” exists that poses a serious threat to U.S. interests and is beyond the capability of civilian law enforcement agencies. DoD personnel may assist the Justice Department in enforcing prohibitions regarding biological or chemical weapons of mass destruction.