Forty-five dead patients have been found in a New Orleans hospital, in the biggest discovery of bodies yet.
Staff stayed to look after hospital patients as the conditions worsened
How the patients died is unclear. One hospital official said the heat after the storm was a factor, while another said some were already dead.
Top US emergencies official Michael Brown has resigned following criticism over the response to Hurricane Katrina.
Officials have said they expect rescue efforts in the affected Gulf Coast states to go on for several more days.
Emergency crews are now reaching outlying areas of Louisiana and Mississippi, as well as the New Orleans neighbourhoods worst hit by flooding.
The BBC's Daniel Lak in New Orleans says the sight of receding waters is starting to tempt a few residents back, despite warnings from the authorities over the health risks.
Business owners are particularly keen to find out what must be done before they can re-open, he says.
The city's airport is due to re-open for commercial flights on Tuesday, in a small sign of progress.
A house-by-house search for the dead continues in New Orleans and elsewhere. The overall confirmed death toll in the hurricane-affected states is now above 500.
Officials have said the final number of dead could be much lower than the thousands feared initially.
The retrieval of 45 patients' bodies from the Memorial Medical Center in New Orleans is under way, health officials said.
Hospital administrator Dave Goodson told the BBC News website he was "disappointed" by a lack of co-ordination in official efforts to evacuate the 317-bed facility.
Staff made heroic efforts to care for the patients who remained in the days after the storm, he said. But heat played "a major factor" in the deaths.
However a spokesman for the hospital's owner, Tenet Healthcare Corp, told the Associated Press some patients were dead before the storm arrived.
None of the deaths resulted from a lack of food, water or electricity to power equipment, Steven Campanini said. Many of the patients were seriously ill.
Volley of criticism
News of Mr Brown's resignation as director of the Federal Emergency Management Agency (Fema) came after President George W Bush visited central New Orleans for the first time since the disaster.
Michael Brown had faced growing criticism
Mr Brown has been replaced at the helm of the hurricane recovery operation by US Coast Guard Vice Admiral Thad Allen.
US media and opposition politicians said Mr Brown lacked disaster expertise, and criticised Mr Bush for appointing him in 2001.
Mr Bush's third visit to the disaster zone came as a Newsweek poll gave Mr Bush his lowest approval rating as president - just 38%.
The president said hard work lay ahead, but people were starting to think about "the long-term revival" of their city of New Orleans.