Forty-five bodies found at the Memorial Medical Centre after the disaster will be tested for toxic substances. Euthanasia is illegal and doctors could be charged with manslaughter if the claims are substantiated.

The mercy killing claims were based on an interview with a doctor from the hospital who said that he saw staff entering wards with syringes after a discussion about euthanasia. Bryant King told CNN that he did not see any patient being injected but was suspicious after hearing a conversation between a hospital official and another doctor, who suggested that patients be put out of their misery.

Dr King said: “One of the other physicians — not the one who had the conversation with me but another — had a handful of syringes. I don’t know what’s in the syringes . . . The only thing I heard her say is, ‘I’m going to give you something to make you feel better.’ ”

He was evacuated from the hospital shortly afterwards and said that he did not know what happened next.

Frank Minyard, the Orleans Parish Coroner, said that investigators had told him that “they thought someone was going around injecting people with some lethal medication”.

The incident, which took place three days after Hurricane Katrina hit and as rising floodwaters made hospital care almost impossible, is under investigation by the state attorney-general.

Tenet Healthcare, which owns the hospital, told CNN that many of the patients who died were critically ill. Eleven patients found in the mortuary had died the weekend before the hurricane, it said.

But Dr King said: “Only one person died overnight. The previous day there were two. For there to be, from Thursday to Friday, ten times that many just does not make sense.”

The euthanasia allegations came as Ray Nagin, the Mayor, began a tour of emergency shelters as the cost of housing the homeless in hotels reached $11 million (£6.27 million) a day, a figure that could rise to $425 million by the October 24 deadline set by President Bush to empty emergency shelters.

Mr Nagin’s message is that New Orleans is ready for its residents to return. Streets are dry, restaurants and offices open, and some schools will begin lessons next month.

But without a critical mass of employees to run the shops, restaurants and amenities, Mr Nagin knows that many thousands will refuse even to contemplate a return.

Some estimates put the num- ber of uninhabitable homes at 200,000, with a disproportionate amount in the poorest areas of the city where evacuees have no resources to rebuild.

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