With pumping operations underway to drain floodwaters from New Orleans and evacuations slowing down, officials braced Tuesday for the grim task of recovering the bodies of people killed by Hurricane Katrina.

New Orleans Mayor Ray Nagin said Tuesday morning that the death toll was likely to horrify the nation. He has estimated that thousands of people died when they were trapped in their homes by the rising floodwaters, but with much of the city still flooded eight days after the hurricane struck, relatively few bodies have been recovered so far.

In Washington, President Bush said he would lead an investigation into the response to Hurricane Katrina, and the Senate's Committee on Homeland Security and Governmental Affairs announced hearings to probe failures in dealing with the storm and its aftermath.

"What I intend to do is . . . to lead an investigation to find out what went right and what went wrong," Bush said after meeting with his Cabinet in the White House. "It's very important for us to understand the relationship between the federal government, the state government and the local government when it comes to a major catastrophe." He said Americans "still live in an unsettled world" and that the government must be able to respond properly if the country is struck by a massive terrorist attack or another major storm.

Sen. Susan M. Collins (R-Me.), who chairs the Senate's homeland security committee, said, "We would be remiss . . . if we did not ask the hard questions needed to understand what went so wrong and what our country must do to improve our ability to respond to future crises, whether they are natural disasters or terrorist attacks."

She said, "It immediately occurred to me that if our system did such a poor job when there was no enemy, how would the federal, state and local governments have coped with a terrorist attack that provided no advance warning and that was intent on causing as much death and destruction as possible?"

An estimated 140,000 to 160,000 homes in New Orleans and hurricane-affected areas of Louisiana have been destroyed by Katrina, the state's environmental chief said Tuesday, and it will take years to restore the water supply to the city and its suburbs.

In a first assessment of the environmental devastation that has struck New Orleans and its surroundings, Mike McDaniel, secretary of Louisiana's Department of Environmental Quality, listed a series of environmental woes that have overwhelmed the ruined city and other areas affected by Katrina.

"It is almost unimaginable what we're going to have to plan for and deal with," said McDaniel.

He said two major oil spills sent 78,000 barrels of oil into Lake Pontchartrain, whose waters flooded into New Orleans through breaches in protective levees. In addition, 2,200 flooded underwater fuel tanks and leaking fuel from cars and boats in the city have contributed to an environmental nightmare.

"Everywhere we look, there are spills," said McDaniel. "It all adds up to almost a solid sheen over the area right now."

In the inundated city, the U.S. Army Corps of Engineers began pumping out floodwaters after plugging a major breach in one of the levees that protect the city.

"The water is flowing the right way now," Lt. Gen. Carl Strock, commander of the Corps of Engineers, said on MSNBC. He said that with a combination of pumping and "gravity draining" through deliberately created breaches in certain levees, "we're achieving success." Some parts of the city could take 24 days to drain, while more difficult areas could take as long as 80 days," Strock said.

In addition to pumps in the city's existing system, Strock said, the corps is bringing in high-capacity pumps from such countries as Germany and the Netherlands.

Nagin, interviewed on CBS's "Early Show," said, "The next immediate step is continue rescue missions, seek and rescue, and then start to extract dead bodies." He has estimated there could be "thousands" of dead in New Orleans.

"Some computer model said 10,000," he added. "I don't know what the number is, but it's going to be big, and it's going to shock the nation."

Asked "who bears the shame" for the inadequate initial response, Nagin said, "I think we all do. I think this is America and what happened here should never, ever happen again in this country."

Nagin said the system needs to be fixed "because too many people suffered both during the storm, but way too many suffered after, and they suffered, and they died, and it didn't have to happen."

The mayor later told reporters that a breach in the 17th Street Canal levee "has been completely repaired." He said that during a helicopter flight over New Orleans Tuesday, "I did not see any water coming in the city. . . . And as a matter of fact, I saw water levels drop significantly in certain areas of the city."

Citing health risks, Nagin urged residents still trying to hold out to accept offers to evacuate.

"There are toxins in the water," he said, "There are gas leaks where we may have explosions. We're fighting at least four fires right now, and we don't have running water. It is not safe. The mosquitoes that are biting dead people are starting to fly. So this is not a safe environment."

He said he understood people's reluctance to abandon their city. But he pleaded, "Let us get you to a better place. Let us clean the city up."

Nagin denied reports that he told the National Guard to withhold food and water to people who refused to evacuate. He said that on the contrary, he replied to such a suggestion from the National Guard by telling officials, "Do not harm anyone, do not allow anyone to starve, do not allow anyone to go without water. . . ."

The deputy police superintendent of New Orleans, Warren J. Riley, estimated that fewer than 10,000 people remain in the city out of a population that stood at 485,000 before a mandatory evacuation ordered the day before Katrina struck at dawn on Aug. 29 with 140-mph winds and torrential rains.

At the city's airport, more than 14,000 evacuees have left on charter flights to various destinations since Saturday, a U.S. Air Force spokesman said. But the numbers of evacuees are steadily dropping, and Tuesday morning the terminal stood practically empty after 472 people left in the early hours.

The evacuees are being taken to locations in downtown New Orleans, then brought by military helicopters to the airport for the charter flights out.

Troops operating under a military unit called Joint Task Force Katrina have turned the airport into the focal point for getting people out of the city. They have ferried in tons of food, water and other supplies and have set up a tent city for service members behind the cargo area.

At a Pentagon briefing, Air Force Gen. Richard B. Myers, chairman of the Joint Chiefs of Staff, said more than 58,000 active duty military and National Guard personnel "are on the ground and in the area" to assist with relief and recovery efforts. He said 17,000 are active duty military from the Army's 82nd Airborne Division and 1st Cavalry Division, the 1st and 2nd Marine Expeditionary Forces and the U.S. Navy. In addition, 4,000 Coast Guard personnel are providing support, Myers said.

The military has deployed 21 naval ships, 350 helicopters and more than 75 fixed-wing aircraft in the effort, he said.

In a Capitol Hill news conference to announce the Senate oversight hearings, Sen. Collins said, "In its initial response to the catastrophe of Hurricane Katrina, particularly in Louisiana, governments at all levels failed in this obligation. The result was enormous suffering, needless deprivation and widespread destruction. . . . It is difficult to understand the lack of preparedness and the ineffective initial response to a disaster that had been predicted for years and for which specific dire warnings had been given for days."

Calling the initial response "woefully inadequate," she said her committee, which has jurisdiction over the Federal Emergency Management Agency, has "initiated an oversight review of the response to Katrina." However, the committee is "determined not to divert resources from the rescue and recovery efforts which are still at a critical stage," she said.

"This is a moment of national crisis," said the committee's ranking Democrat, Sen. Joseph I. Lieberman of Connecticut. He said the hurricane attacked not only the Gulf Coast, but "America's self-confidence" in its ability to respond to emergencies.

"This is no time for politics or partisanship," he said. The hearings would ensure that the mistakes in the aftermath of Katrina were not repeated, Lieberman said.

Bush, speaking to reporters after the Cabinet meeting, said he has asked Vice President Cheney to visit the Gulf Coast on Thursday "to assess our recovery efforts" and help determine whether the government is meeting key goals.

Bush said, "Our immediate goals are these: We want to complete the search and recovery; we want to restore essential services; we want to drain the water in the affected areas and begin removing debris; we want to -- and all are assessing public health and safety matters."

Cheney will work with Homeland Security Secretary Michael Chertoff and others to remove any "bureaucratic obstacles," Bush said. "In other words, bureaucracy is not going to stand in the way of getting the job done for the people."

The president also emphasized that the federal government would ensure that displaced victims of Katrina receive Social Security checks and other benefits they are due.

Asked if he intends to replace any administration officials in view of the widespread criticism of the federal response, Bush did not answer directly, but said he would lead an investigation.

"We want to make sure that we can respond properly if there's a WMD [weapons of mass destruction] attack or another major storm," Bush said. "And so I'm going to find out over time what went right and what went wrong." He did not immediately provide any details of the probe.

Bush indicated that he did not want to "play a blame game." He said, "We've got to solve problems. We're problem-solvers. There will be ample time for people to figure out what went right and what went wrong."

Branigin reported form Washington.