The POTUS declared an emergency and authorized FEMA to do whatever was necessary back on August 27th.
http://www.whitehouse.gov/news/releases/2005/08/20050827-1.html
I don't buy the argument that they couldn't do anything because they weren't asked in time. Nor does it help to know that the dems carry at least as much of the blame. The response to Katrina was inept and criminal from the top down.