An investigation into mercy killings during the chaos after Hurricane Katrina has led to the arrests of a doctor and two nurses who allegedly administered lethal doses of morphine and another drug to four New Orleans hospital patients.

The deaths at Memorial Medical Center, where 34 patients died in the days after Katrina, originally came to the attention of authorities described as euthanasia, but Louisiana Attorney General Charles C. Foti said yesterday that the killings were murder.

"We're talking about people that pretended that maybe they were God," Foti said during a news conference in Baton Rouge. "This is not euthanasia. This is plain and simple homicide."

Anna Pou, an ear, nose and throat specialist, and nurses Lori L. Budo and Cheri A. Landry, were arrested late Monday. Each was charged with four counts of being a principal in a second-degree murder.

Exactly who administered the doses is not clear, and as the matter moves into the legal system, its outcome seems likely to turn on the desperation that had enveloped the hospital, the motives of Pou and the two nurses, and whether the four patients would have lived to be rescued if the alleged lethal doses had not been administered.

As many as 14 of the bodies found at the hospital showed indications that a lethal dose of drugs had been administered, authorities said, but only four of the deaths led to cases strong enough to present to a judge.

In the flooded city, some of the greatest desperation was at hospitals and nursing homes where frail people were left without air conditioning, medicine and basic care. Doctors pleaded for helicopters and boats and feared looters. Amid criticism that patients should have been evacuated beforehand, health-care authorities have praised the "heroic" actions to save and comfort stranded patients. Some patients were simply too weak to transport, they said, and it was deemed better to stay open during a hurricane to cater to storm victims.

For several months, Foti has been investigating evacuation decisions and patient care during the storm at 13 nursing homes and five hospitals, and yesterday's announcement was the second major set of arrests to emerge from the probe. Last year, the owners of a nursing home in St. Bernard Parish were charged with negligent homicide because 34 people in their care perished in the flooding. That case is pending.

Pou's attorney, Rick Simmons, said there was no basis for murder charges. "My client worked there feverishly for five days to comfort and evacuate patients," he said.

Temperatures inside Memorial Medical Center rose to 100 degrees, the generators didn't work, toilets backed up, and nurses had to resort to improvising care. Rescues were sporadic. The case against Pou and the others began after attorneys for Lifecare Hospitals, an acute-care company that leases a portion of the hospital, reported "the possible euthanasia of patients following Hurricane Katrina," according to the arrest affidavit. On Sept. 1, Pou told three Lifecare administrators that "a decision had been made to administer lethal doses" to some patients, the affidavit said.

It said Pou also showed the Lifecare pharmacy director a pack of 25 vials of morphine and requested supplies such as syringes.

One of the four patients, obese and paralyzed, was nonetheless alert and conscious; another said "that burns" when the dose was given, according to accounts in the affidavit.

Test results on the four bodies listed in the affidavit were positive for morphine and midazolam, the generic name for a drug known as Versed, and none of the patients was receiving those drugs for their routine care.

Foti would not say why Pou and the nurses would have killed the patients.

"It is not my job or duty to say what the motive was," he said. He did not publicly identify the four dead patients.

But he said there was no indication that the patients had asked to be killed or that any were about to die as the hospital descended into crisis. "I think the patients would have lived through it," he said.

But Simmons said the motive in a homicide is critical, and Foti's failure to outline one could unravel the case. The district attorney in New Orleans will now consider the evidence and decide whether to bring formal charges.

Foti "says he couldn't figure out a motive," Simmons said. "But if he can't figure out a motive, how can he prove a homicide? A homicide involves intentional actions."

Angela McManus had been living at her mother's bedside until ordered to leave and believes that her mother was among those administered a lethal dose. "It's a relief," she said of the arrests. "She might still be here if they had simply rescued her."

In a statement, Tenet Healthcare, which owns the medical center, said, "Euthanasia is repugnant to everything we believe as ethical health-care providers, and it violates every precept of ethical behavior and the law. It is never permissible under any circumstances."