I suspect a number of the gangsters were killed at night by elite military units who acted without publicity and without the knowledge or consent of city and state officials. The media were basically confined to the downtown area and reported on the crime and misery in the Super Dome or the "Southern Decadence" party by a handful of diehard homosexuals in the French Quarter. The reason I suspect this was the case is because you did not hear much about the gangs and sniping or looting after last Wednesday night. With so many dead bodies around, a few dozen more with bullet holes will not be noticed, especially after a few weeks floating in the sewage waters. Any witnesses who survive will lack any photo or video evidence of the operation, and their testimony will only be believed by the fringe types at Democratic Underground or like minded places.
What was probably done by the elite units was necessary, yet their actions did not adhere to the letter of the law. However, the Constitution is not a suicide pact and there are enough realists in the Bush Administration who know what is the right thing to do.
A number of National Guard units reported that the positions they had been sniped from the previous couple of days had suddenly been abandoned. When they went to examine the sites, they found abandoned weapons still loaded and usable, and possible signs of a struggle (at many locations). No bodies and no other evidence, though.
Oddly enough, posse commitatus does not apply to private contractors working for the US Government outside the command of the military.