There is a federal law, posse comitatus (sp?), that prohibits the military from interfering in domestic law enforcement issues. If you remember during the DC sniper shootings, local law enforcement asked the Pentagon for help and there was a lot of reluctance to provide any. I am not 100% sure, but I believe the President can waive this under certain circumstances. If you are wondering, this law does not apply to National Guardsmen because they are under the control of state governors.