More than once this law has been cited to explain the DOD's seeming and actual inaction. If you read the law, there's not a whole lot of wiggle room in it.
The Wikipedia Posse Comitatus Article.Sec. 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.