That's the Posse Commitatus Act. Active duty military are forbidden form exercising police power against US citizens on US soil.
But, active duty military can do anything else, e.g., search and rescue (and of course, self defense). And, Nat'l Guard can perform the police function if the state or local authorities are outgunned by the thugs.
It was reported in the WP that Blanco refused to sign a memorandum so the Feds/Military could take control of the Citie/State
I wonder if the media/press will ask these questions to Blanco???
Some how I doubt they will