An explanation of the Posse Comitatus Act from Rand.org:
https://www.rand.org/content/dam/rand/pubs/monograph_reports/MR1251/MR1251.AppD.pdf
More importantly:
"KEY EXCEPTIONS TO THE POSSE COMITATUS ACT
A summary of key exceptions to the Posse Comitatus Act follows:3
• National Guard forces operating under the state authority of Title 32 (i.e., under state rather than federal service) are exempt from Posse Comitatus Act restrictions.
• Pursuant to the presidential power to quell domestic violence, federal troops are expressly exempt from the prohibitions of Posse Comitatus Act, and this exemption applies equally to active-duty military and federalized National Guard troops.4
• Aerial photographic and visual search and surveillance by military personnel were found not to violate the Posse Comitatus Act.
• Congress created a “drug exception” to the Posse Comitatus Act. Under recent legislation, the Congress authorized the Secretary of Defense to make available any military equipment and personnel necessary for operation of said equipment for law
enforcement purposes. Thus, the Army can provide equipment, training, and expert military advice to civilian law enforcement agencies as part of the total effort in the “war on drugs.”
• Use of a member of the Judge Advocate Corps as a special assis- tant prosecutor, while retaining his dual role in participating in the investigation, presentation to the grand jury, and prosecution, did not violate Posse Comitatus Act.
• The Coast Guard is exempt from Posse Comitatus Act during peacetime.
• Although brought under the Act through DoD regulation, described above, the Navy may assist the Coast Guard in pursuit, search, and seizure of vessels suspected of involvement in drug trafficking."