Florida Governor Rick Scott has vowed his state will aggressively investigate how six people died at a nursing home that lost power when Hurricane Irma rampaged through the region, as millions manage another day without electricity.
The death toll from the storm approached 80 on Wednesday as officials continued to assess the damage after Irma powered through the Caribbean as one of the most powerful Atlantic hurricanes on record and slammed into the Florida Keys archipelago with sustained winds of up to 215 km/h.
Irma killed at least 36 people in Florida, Georgia and South Carolina. Some 4.2 million homes and businesses, about 9 million people, were without power.
Police opened a criminal investigation at the Rehabilitation Center at Hollywood Hills in Hollywood, north of Miami, where three elderly residents were found dead at the facility and three later died at a nearby hospital, officials said.
"I am going to work to aggressively demand answers on how this tragic event took place," Scott said in a statement. "This situation is unfathomable. Every facility that is charged with caring for patients must take every action and precaution to keep their patients safe."
More than 100 patients at the nursing home were evacuated along with 18 patients from a nearby facility that was cleared due to the criminal investigation, Hollywood officials said.
"Most of the patients have been treated for respiratory distress, dehydration and heat-related issues," said Randy Katz, a spokesman for Memorial Regional Hospital.
The centre had been without air conditioning.
Florida Power & Light said the facility was not on a county top tier list for emergency power restoration.
Irma caused about $US25 billion ($A31 billion) in insured losses, including $US18 billion in the United States and $US7 billion in the Caribbean, catastrophe modeller Karen Clark & Co estimated.
The Florida Keys were particularly hard hit, with federal officials saying that 25 per cent of homes were destroyed and 65 per cent suffered major damage when Irma barrelled ashore on Sunday as a Category 4 hurricane.
Irma wreaked total devastation in parts of the Caribbean, where at least 43 people have died.
People who fled their homes in hard-hit islands including St Martin and the US Virgin Islands that were all but cut off from the world for days arrived in San Juan late Tuesday.
Irma hit the United States about two weeks after Hurricane Harvey hit Houston, killing about 60 and causing some $US180 billion in damage, mostly from flooding.