"The people that I met in Houston — they want answers and they want to know what went wrong and they want to know what they are going to be able to count on in the future," she said on NBC's "Today" show Wednesday, two days after visiting refugees at the Astrodome. "I don't think the government can investigate itself."

The pumping-out of the city began after the Army Corps of Engineers used hundreds of sandbags and rocks over the Labor Day weekend to close a 200-foot gap in the 17th Street Canal levee that burst in the aftermath of the storm and swamped 80 percent of this below sea-level city.

Although toxic floodwaters receded inch by inch, only five of New Orleans' normal contingent of 148 drainage pumps were operating Tuesday, the Corps said.

How long it takes to drain the city could depend on the condition of the pumps — especially whether they were submerged and damaged, the Corps said. Also, the water is full of debris, and while there are screens on the pumps, it may be necessary to stop and clean them from time to time.

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Associated Press writers Doug Simpson, Dan Sewell, Jim Litke, Melinda Deslatte, Matt Apuzzo and Randolph E. Schmid contributed to this report.