>>good read on posse comitatus act

https://www.everycrsreport.com/reports/R42659.html

the courts have held that, absent a recognized exception, the Posse Comitatus Act is violated when (1) civilian law enforcement officials make “direct active use” of military investigators; or (2) the use of the military “pervades the activities” of the civilian officials; or (3) the military is used so as to subject “citizens to the exercise of military power which was regulatory, prescriptive, or compulsory in nature.” The act is not violated when the Armed Forces conduct activities for a military purpose.

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Questions concerning the act’s application arise most often in the context of assistance to civilian police. At least in this context, the courts have held that, absent a recognized exception, the Posse Comitatus Act is violated when (1) civilian law enforcement officials make “direct active use” of military investigators; or (2) the use of the military “pervades the activities” of the civilian officials; or (3) the military is used so as to subject “citizens to the exercise of military power which was regulatory, prescriptive, or compulsory in nature.” The act is not violated when the Armed Forces conduct activities for a military purpose.

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The language of the act mentions only the Army and the Air Force, but it is applicable to the Navy and Marines by virtue of administrative action and commands of other laws. 

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Although violations will on rare occasions result in the exclusion of evidence, the dismissal of criminal charges, or a civil cause of action, as a practical matter compliance is ordinarily the result of military self-restraint.