The nursing home deaths brought the total number of storm-related fatalities in Florida to 20 and illustrated the urgency of restoring electricity to millions of people across the southern state.
Around 115 residents of the retirement home in Hollywood, north of Miami, were immediately evacuated after a nearby hospital began receiving patients suffering from heat-related problems.
Three of those who died were in their nineties, including 99-year-old Albertina Vega. The youngest was 70, according to the Broward County Medical Examiner's office.
Governor Rick Scott said he was "absolutely heartbroken" to learn of the deaths of the elderly retirees.
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"I am going to aggressively demand answers on how this tragic event took place," Scott said in a statement. "Although the details of these reported deaths are still under investigation, this situation is unfathomable."
The governor said he has ordered a probe by state authorities, and Sanchez confirmed a criminal investigation has been launched.
Memorial Regional Hospital began receiving patients early yesterday, said Dr Randy Katz, medical director of the hospital's emergency department.
"(We) quickly identified some issues inside the facility with fire rescue and immediately evacuated the building," Katz said.
He said most of the patients admitted to hospital have been treated for respiratory distress, dehydration and heat- related issues.
Florida officials have made restoring lost power a priority and tens of thousands of utility company workers are engaged in the huge effort, many from out of state.
President Donald Trump and his wife, Melania, were to visit Florida to survey the damage today.
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