High on the seventh floor of Memorial Medical Center, some of the city’s sickest patients lay in wet, sweaty sheets, drifting in and out of consciousness.

In the hours before the last of the hospital’s patients were evacuated, one of Hurricane Katrina’s most uncomfortable decisions had to be made: What would happen to those too sick to be moved?

According to a monthslong investigation by the state’s attorney general, a doctor and two nurses “pretended that maybe they were God” and put to death four patients using a lethal injection of drugs, after determining that the four were either too ill or too incapacitated to be transported.

The deceased, who ranged in age from 61 to 90 years old, would have survived Katrina had they not been administered the lethal doses, Louisiana Attorney General Charles C. Foti said.

On Tuesday, a doctor and two nurses were booked with being “a principal to second-degree murder” — a charge that carries a mandatory sentence of life in prison.

The arrest of Dr. Anna Pou, and nurses Cheri Landry and Lori Budo, brings to an end months of speculation about so-called “mercy killings” at Memorial. Although attorneys for the three insist no crime took place, the three women are the first medical professionals charged in a 10-month-long criminal investigation into whether many of New Orleans’ sick and elderly were abandoned or put out of their misery in the days after the storm.

Foti, who late last year subpoenaed more than 70 employees of the hospital, warned that there will likely be more arrests and more victims identified: “This case is not over yet,” he said. And he stressed: “This is not euthanasia. This is homicide.”

At least 34 patients died at the eight-story, 317-bed hospital, which became a death trap as temperatures soared past 100 degrees and floodwaters cut off access to the building in the chaotic days following Katrina.

In court papers, state investigators said Pou told a nurse executive three days after the hurricane that the patients still awaiting evacuation would probably not survive and that a “decision had been made to administer lethal doses” to them. According to Foti, Pou and the others filled syringes with a deadly combination of morphine and a sedative, called Versed, carefully flushing water into the patient’s IV to ensure that the drugs fully entered the bodies of the sick.

According to court papers, tissue samples taken from the dead at Memorial tested positive for morphine and Versed, and the amount of Versed present was found to be higher than the usual therapeutic dose. Medical records reviewed by investigators also showed that none of the four patients was taking either of the two drugs as part of their routine care.

Foti said the combination of the two drugs “guarantees they are going to die,” but he added that authorities could not determine which of the defendants actually administered the fatal drugs in each case.

According to an affidavit filed by an investigator for the state, some of the patients experienced pain. A hospital director witnessed one of the nurses injecting a 90-year-old patient. As she did, the patient said: “That burns.”

In another instance, Pou told a hospital administrator that she was “under the impression that the patients were not aware of what was happening.” That staffer corrected her, pointing out that one patient — a 61-year-old man weighing 380 pounds and paralyzed — appeared “conscious, awake and alert.” So, according to the affidavit, she asked for the man to be sedated.

Those who knew Pou struggled to make sense of the arrest. Pou was handcuffed in her scrubs in front of her home late Monday night, according to her mother and her lawyer.

Pou, said Dr. Gregory Vorhoff, who was at the hospital during the storm, is “an extremely bright and capable physician.

“I would never hesitate to have those nurses or Anna Pou take care of me or my family.”

“Medicine was the most important thing in her life and I know she never ever did anything deliberately to hurt anyone,” added Jeanette Pou, the doctor’s mother, in a telephone interview.

News of the arrests was met with relief by the families of the dead.

Angela McManus was forced to leave her 70-year-old mother on the acute-care floor of Memorial Hospital Sept. 1 when police demanded relatives evacuate. McManus’ mother was not among the four the doctor and nurses are accused of killing. She said her mother, who was recovering from a blood infection, was fine when she left her, then mysteriously died the same day.

“I need some answers just to be able to function,” said McManus. “Euthanasia is something you do to a horse, or to an animal. When you do it to people, it’s called murder.”