HOLLYWOOD, Fla. — The first patient was rushed into the emergency room of Memorial Regional Hospital around 3 a.m. on Wednesday, escaping a nursing home that had lost air-conditioning in the muggy days after Hurricane Irma splintered power lines across the state.

Another arrived at 4 a.m. After a third rescue call, around 5 a.m., the hospital’s staff was concerned enough to walk down the street to check the building themselves.

What they found was an oven.

The Rehabilitation Center at Hollywood Hills needed to be evacuated immediately. Rescue units were hurrying its more than 100 residents out. Dozens of hospital workers established a command center outside, giving red wristbands to patients with critical, life-threatening conditions and yellow and green ones to those in better shape.

Checking the nursing home room by room, the hospital staff found three people who were already dead and nearly 40 others who needed red wristbands, many of whom had trouble breathing. The workers rushed them to Memorial’s emergency room, where they were given oxygen. The rest went to other hospitals nearby.

Four were so ill that they died soon after arriving. In the afternoon, the authorities learned that another had died early in the morning, and was initially uncounted because the person had been taken directly to a funeral home.

In all, eight were dead.

“We had no idea the extent of what was going on until we literally sent people room to room to check on people,” said Dr. Randy Katz, the hospital’s chairman of emergency medicine.

Three days after the hurricane had howled through South Florida, some of the most vulnerable people in the state were dying, not of wind, not of floods, but of what seemed to be an electrical failure.

Florida was still staggering to its feet on Wednesday, and millions of people across the Southeast were facing days or weeks without power in temperatures that, in the Fort Lauderdale area, climbed to as high as 92 degrees in recent days. The nursing home appeared to have electricity, but the hurricane had knocked out power in a critical spot: A tree had apparently hit the transformer that powered the cooling system, intensifying the subtropical heat from oppressive to fatal.

State officials, utility executives and the Rehabilitation Center spent Wednesday trading blame over why and how its patients were left to endure such conditions, even though state and federal regulations require nursing home residents to be evacuated if it gets too hot inside.

The Hollywood Police Department opened a criminal investigation into the deaths of the eight residents, who ranged in age from 71 to 99, and investigators from the state attorney general’s office were also involved. Gov. Rick Scott ordered a moratorium on admissions at the nursing home.

By day’s end, the unanswered questions were still outstanding, even as the deaths magnified scrutiny on other facilities for the old and disabled.

More than three million customers in Florida still lacked power Wednesday, including roughly 160 nursing homes, according to the state’s tracking system. After generators fizzled at the Krystal Bay Nursing and Rehabilitation Center, in North Miami Beach, 79 people were evacuated as a precaution.

“I am going to aggressively demand answers on how this tragic event took place,” Mr. Scott said in a statement. “Although the details of these reported deaths are still under investigation, this situation is unfathomable. Every facility that is charged with caring for patients must take every action and precaution to keep their patients safe — especially patients that are in poor health.”

Dr. Katz said Memorial’s emergency room had been busy for days treating chronically ill patients who were not coping well with the loss of electricity; some were having trouble breathing in the heat, while others needed access to dialysis. At least one came in from the Rehabilitation Center on Tuesday.