With thousands feared dead and the city's remaining residents told to evacuate for weeks, conditions deteriorated further in submerged New Orleans as looting spiraled out of control.
Mayor Ray Nagin ordered virtually the entire police force to abandon search-and-rescue efforts and stop thieves who were becoming increasingly hostile.
"They are starting to get closer to heavily populated areas -- hotels, hospitals, and we're going to stop it right now," Nagin said Wednesday.
Tempers also were starting to flare. Police said a man in Hattiesburg, Mississippi, fatally shot his sister in the head over a bag of ice. Dozens of carjackings were reported, including a nursing home bus and a truck carrying medical supplies for a hospital. Some police officers said they had been shot at.
Earlier Wednesday, Nagin called for a total evacuation, saying that New Orleans will not be functional for two or three months and that people would not be allowed back into their homes for at least a month or two.
The first of nearly 25,000 refugees being sheltered at the city's Superdome football stadium were transported in buses to the another stadium in Houston, Texas, 560km away. Conditions in the Superdome had become horrendous: There was no air conditioning, the toilets were backed up, and the stench was so bad that medical workers wore masks as they walked around.
Asked how many people died in the hurricane, Nagin said: "Minimum, hundreds. Most likely, thousands." The death toll has already reached at least 110 in Mississippi.
If the mayor's death-toll estimate holds true, it would make Katrina the worst natural disaster in the US since at least the 1906 San Francisco earthquake and fire, which have been blamed for anywhere from about 500 to 6,000 deaths. Katrina would also be the nation's deadliest hurricane since 1900, when a storm in Galveston, Texas, killed between 6,000 and 12,000 people.
Just outside New Orleans, gunmen held up a supply truck carrying food, water, medical supplies and pharmaceuticals, prompting officials to ask police and the US Coast Guard to help evacuate a 203-bed hospital.
"We have to close it down because we can no longer ensure the safety of our patients or our staff in that hospital," said Steven Campanini, a spokesman for Tenet Healthcare Corp.
He said there were about 350 employees and between 125 to 150 patients inside the hospital, which is not flooded and is functioning.
Looters used garbage cans and inflatable mattresses to float away with food, clothes, TV sets -- even guns. Outside one pharmacy, thieves commandeered a forklift and used it to push up the storm shutters and break through the glass. The driver of a nursing-home bus surrendered the vehicle to thugs after being threatened.
US President George W. Bush flew over New Orleans and parts of Mississippi's hurricane-blasted coastline in Air Force One. Turning to his aides, he said: "It's totally wiped out. ... It's devastating, it's got to be doubly devastating on the ground."
"We're dealing with one of the worst national disasters in our nation's history," Bush said later in a televised address from the White House, which most victims could not see because power remains out to 1 million Gulf Coast residents.
He planned to appear on ABC's Good Morning America program Thursday to discuss the tragedy and recovery efforts.