Here, in question-and-answer form, is a look at the disaster.
How well prepared was New Orleans for such a storm?
The city is especially vulnerable because much of it sits below sea level. Local, state and federal officials have worked for years preparing plans for evacuation and local shelter. The giant Superdome was first used as a shelter in 1998 when Hurricane Georges threatened the city. Just last summer, officials conducted an exercise to prepare for a major hurricane in New Orleans, concluding that as many as a million people might be forced from their homes and that some shelters would be needed for as long as 100 days.
Why wasn't everyone evacuated?
A mandatory evacuation was ordered when it became apparent the storm track would take it close to the city. Hundreds of thousands responded, but many didn't have the means to leave. Of those remaining behind, thousands sought shelter in the Superdome. "Sometimes people refuse to be taken away," Federal Emergency Management Agency director Michael Brown said.
Why wasn't aid available sooner?
Food, water and other supplies, as well as search and rescue teams, were brought in in advance of the storm. "In New Orleans we were ready to move in as soon as Katrina moved out," but levee failures then made it unsafe to move into many parts of the city, Brown said.
Was there a shortage of National Guard troops available to assist because so many are serving in Iraq and Afghanistan?
National Guard officials say that was not a problem. On Friday, three days before Katrina's landfall, 10,000 National Guard troops were dispatched across the Gulf Coast. Lt. Gen. Steven Blum, commander of the National Guard, said bringing in more has been hampered by road conditions but he estimated there will be 32,000 National Guard troops in the region by Monday.
Why is government aid being sent by ship instead of by air?
Airports at New Orleans and nearby Gulfport, Miss., were closed because of storm damage. The New Orleans airport has reopened only for humanitarian supply flights, which are landing there in daylight hours. Assistance also is being delivered by trucks, ships and by airplanes landing at other nearby cities.
Why did the levees fail?
According to the Army Corps of Engineers, the levees were designed to withstand a Category 3 storm, the middle level in a system (1 to 5) that measures hurricane intensity. The force of Katrina was more than the levee could withstand, the Corps said. The agency was to begin a feasibility next year on what it would take to protect the city from stronger, category 4 or 5 storms.
How long will it take to drain the city?
A: The Corps of Engineers says it cannot yet estimate the time. It will depend on the weather and how soon repairs to levees are completed. The pumps can then begin sending the water into nearby waterways. The city has pumps and additional ones can be brought in, but electricity is still out so generators will be needed. It's not like pulling the plug on a bathtub drain; much of the city is below sea level so the water will have to be pumped up and out.
Is it true cruise ships will be used for those forced out of their homes?
Both Brown and the cruise ship industry report they are talking about this possibility.