Some 4.2 million homes and businesses, or about 9 million people, were without power on Wednesday in Florida and nearby states.
Irma hit Florida about two weeks after Hurricane Harvey pounded Houston, killing about 60 people and causing some US$180 billion in damage, mostly from flooding.
Elsewhere, at least 13 people are believed to have been killed in Florida in Irma-related circumstances.
"The tragic and senseless loss at Hollywood Hills Rehabilitation Center is the subject of a criminal homicide investigation by law enforcement", the DOH noted. The storm was so large that it spanned approximately 400 miles wide, covering the whole state.
"We're looking into the temperatures inside the facility, the staffing inside the facility and the conditions leading up to these incidents", said Storey who also confirmed that police had received a warrant to search inside the facility.
"Did the system break down somewhere between the nursing home itself and the power company and emergency operations folks?" "This situation is unfathomable".
Some residents there awoke sick, the Miami Herald reported. "Our staff continually checked on our residents' well-being-our most important concern-to ensure they were hydrated and as comfortable as possible".
When FPL finally did arrive at the Hollywood Hills nursing home Wednesday, after the patients had died, it took FPL workers approximately 20 minutes to get the nursing home back up and running. He later met with local officials and first responders in Fort Myers and Naples to hear of the impacts of Hurricane Irma firsthand. The staff used fans, put cold towels and ice on patients and gave them cold drinks, he said.
In addition, there are more than 3,100 assisted living facilities with more than 99,000 beds. People on the west coast, where Hurricane Irma made landfall, will likely have power restored by Friday, according to Florida Power & Light Co.
The association said it was working with officials to prioritise "the locations with the greatest need".
The number of people without electricity in the steamy late-summer heat dropped to 9.5 million - just under half of Florida's population.
Pembroke Pines police spokeswoman Amanda Conwell told the Miami Herald that officers were at the scene.
"So what I'm expecting is going to happen as soon as we're able to open the gates ... is you're going to see the crisis escalate again", Senterfitt said.
The center said the hurricane knocked out a transformer that powered the air conditioning.
At least six people in Florida died of apparent carbon monoxide poisoning from generators, and a Tampa man died after the chain saw he was using to remove trees recoiled and cut his carotid artery.
Florida residents are counting the cost of Irma's destruction.