JP 3-28, page 59

Supporting Civilian Law Enforcement Agencies III-3 authority and actions taken under the Insurrection Act are expressed exceptions to the PCA. These exceptions allow federal forces to perform actual law enforcement functions within the guidance of DODI 3025.21, Defense Support of Civilian Law Enforcement Agencies. This directive states that federal military commanders shall not take charge of any function of civil government unless absolutely necessary under conditions of extreme emergency. Any commander who is directed, or undertakes, to control such functions shall strictly limit military actions to the emergency needs, and shall facilitate the reestablishment of civil responsibility at the earliest time possible. (3) Title 10, USC, Chapter 15, Insurrection. (a) This law authorizes the President to employ the Armed Forces of the United States, including the NG in federal status, within the US to support a request from a state legislature, or its governor if the legislature cannot be convened, to suppress an insurrection; suppress a rebellion against the authority of the US, which makes it impracticable to enforce the laws of the US by the ordinary course of judicial proceedings; and suppress, in any state, any insurrection, domestic violence, unlawful combination, or conspiracy, if it (1) hinders execution of state and US law protecting Constitutional rights and the state is unable, fails, or refuses to protect those rights, thereby denying equal protection of the law secured by the Constitution, or (2) opposes or obstructs execution of US law. Actions under this authority are governed by DODI 3025.21, Defense Support of Civilian Law Enforcement Agencies. (b) The President executes this authority by issuing a proclamation ordering the insurgents to disperse and retire peaceably to their homes within a limited time. Any DOD forces employed in civil disturbance operations shall remain under federal military authority at all times. (c) Forces deployed to assist federal and local authorities during times of civil disturbance follow the rules for the use-of-force found in CJCSI 3121.01, (U) Standing Rules of Engagement/Standing Rules for the Use of Force for US Forces. b. Prohibited Direct Assistance. Direct assistance and participation by military personnel in the execution and enforcement of the law is the heart of the prohibition of the PCA. Impermissible direct assistance by military personnel in civilian law enforcement activities is addressed in DODI 3025.21, Defense Support of Civilian Law Enforcement Agencies. Prohibited direct assistance by military personnel includes, among other activities: (1) Interdiction of a vehicle, vessel, or aircraft. (2) A search or seizure. (3) An arrest, apprehension, stop and frisk, interviews and questioning of potential witnesses, or similar activity. (4) Use of military personnel for surveillance or pursuit of individuals or as undercover agents, informants, investigators, or interrogators.