Posse Comitatus, Q say?

Using Defense Intelligence to spy on Americans without due process would violate the PCA:

The text of the relevant legislation is as follows:

18 U.S.C. § 1385. Use of Army and Air Force as posse comitatus Whoever, except in cases and under circumstances expressly authorized by the Constitution or Act of Congress, willfully uses any part of the Army or the Air Force as a posse comitatus or otherwise to execute the laws shall be fined under this title or imprisoned not more than two years, or both.

Also notable is the following provision within Title 10 of the United States Code (which concerns generally the organization and regulation of the armed forces and Department of Defense):

10 U.S.C. § 275. Restriction on direct participation by military personnel The Secretary of Defense shall prescribe such regulations as may be necessary to ensure that any activity (including the provision of any equipment or facility or the assignment or detail of any personnel) under this chapter does not include or permit direct participation by a member of the Army, Navy, Air Force, or Marine Corps in a search, seizure, arrest, or other similar activity unless participation in such activity by such member is otherwise authorized by law.

Thank you, Wikipedia. Note well: "search, seizure, arrest, or other similar ac activity.