In Washington, President George W. Bush vowed to lead a probe into the government response to the disaster which a senior Republican lawmaker criticized as "woefully inadequate."

As the waters slowly receded in New Orleans, partly exposing buildings that had been covered to their rooftops, Nagin said the country should brace for some shocking images.

But the mayor also spoke of "rays of light" returning to the devastated jazz capital as it began to show signs of life after a week of destruction, violence and despair.

Convoys of supply trucks, ambulances, buses and armored personnel carriers produced the first downtown traffic jam since Katrina struck. A few suburban streetlights came on and at least one pharmacy and several gas stations opened.

Helicopters buzzed overhead and an ad hoc fleet of rescue boats, private and public, plied the putrid waters searching for survivors but mainly finding bloated bodies floating freely or tethered to lampposts.

Three of the city's 148 permanent pumps were up and running and army engineers were flying in high-capacity pumps from Germany and the Netherlands to bring the water level down and clear the way for cleanup operations.

Authorities estimated it would take between 36 and 80 days to drain the city and painted an apocalyptic picture of Katrina's environmental legacy with the sewage system in tatters, highly contaminated water supplies and oil slicks threatening wildlife.

"It is almost unimaginable the things we are going to have to plan for and deal with," said Mike McDaniel, secretary of the Louisiana Department of Environmental Quality.

The pumps went into action after engineers succeeded in closing a massive hole in a levee that broke under Katrina's force and unleashed the surging waters of Lake Pontchartrain on the city.

The massive round-the-clock project saw contractors drive piles into the gap and huge twin-rotor Chinook helicopters dump hundreds of bags full of sand, cement and bits of torn-up roadway.

Bush, meanwhile, remained on the defensive over his administration's handling of Katrina, amid widespread criticism that it was slow to speed troops and relief supplies after the storm hit on August 29.

The president said after a cabinet meeting that the situation boded ill for the country's preparedness to deal with any terrorist attack.

"What I intend to do is to lead an investigation to find out what went right and what went wrong," he told reporters.

Bush, who made his second tour of the devastation Monday, said Vice President Dick Cheney would visit the area Thursday to assess recovery efforts and cut any bureaucratic red tape.

But he rejected calls to fire officials such as embattled Federal Emergency Management Agency (FEMA) head Mike Brown, saying he would focus on dealing with the problems at hand rather than "play a blame game."

Republican Susan Collins, who heads the Senate's Government Reform and Homeland Security Committee, said the response to Katrina had been "woefully inadequate" at all levels of government. "We would be remiss ... if we did not ask the hard questions needed to understand what went so wrong," Collins said.

Following a meeting with Bush, Democratic Senate Minority Leader Harry Reid said the White House was planning to ask for another 40-50 billion dollars to pay for relief and recovery efforts.

Congress approved an initial 10.5 billion dollar emergency aid package in an extraordinary session last week.

The authorities faced some pressing challenges in the days and weeks ahead, notably how to provide shelter, food, medical care and schools for hundreds of thousands of people made homeless by Katrina.

Moved by the suffering, Americans across the United States opened their homes, trailers and vacation getaways. Two cruise ships off the coast of Texas were chartered to provide shelter for some 4,000 people.

The housing problem risked becoming a long-term crisis. Officials here said 140,000 to 160,000 homes swamped in New Orleans were unsalvageable.

The oil industry in the southern United States showed signs of life as authorities strained to get refineries, ports and shipping channels back up.

But Lieutenant General Carl Strock, commander of the Army Corps of Engineers, said the critical task of restoring navigation remained.

"We must get the ports open. Sixty percent of our grain moves through the ports of New Orleans," Strock told CNN.

All rights reserved. � 2005 Agence France-Presse. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by Agence France-Presse. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of Agence France-Presse.

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