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.
"It's a sad event," Hollywood police chief Tomas Sanchez told a news conference. "We believe at this time they (the deaths) may be related to the loss of power in the storm."
Hollywood mayor: 'Widespread' effort to help elderly
Governor Rick Scott said he was "absolutely heartbroken" to learn of the deaths of the elderly retirees.
"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.
"If they find that anyone wasn't acting in the best interests of their patients, we will hold them accountable to the fullest extent of the law," he said.
Sanchez, the police chief, said a criminal investigation had been launched.
The Sun-Sentinel newspaper, citing the Broward County Medical Examiner's Office, said the eight people who died were between the ages of 71 and 99 years old.
Dr. Randy Katz, medical director of the emergency department at Memorial Regional Hospital, said the hospital began receiving patients early on Wednesday.
"(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 the hospital have been treated for respiratory distress, dehydration and heat-related issues.
Florida officials have made restoring electricity to the millions who have lost power a priority and tens of thousands of utility company workers, many from out of state, are engaged in the huge effort.
Heartbreak for some returnees
Florida residents who evacuated ahead of the storm were facing lengthy traffic jams, meanwhile, as they returned to check out their homes after days in shelters or with friends or family.
Irma, which made landfall Sunday morning in the Florida Keys as a Category Four hurricane, had triggered orders for more than six million people to flee to safety, one of the biggest evacuations in US history.
Some residents of the Keys were met with scenes of devastation on Tuesday after police lifted roadblocks and began allowing people to return to the string of islands off Florida's southern coast.
At least a quarter of the homes in the Keys were destroyed, according to federal emergency management officials, and 65 percent suffered some damage.
The storm also cut off power, water and sewage to the islands, which are home to some 70,000 people.
"Bad, it's real bad," Keys resident Bryan Holley told NBC News.
"It's gonna take months, maybe years to get this cleaned up," said Holley, who ignored orders to evacuate and braved the storm on Cudjoe Key.
President Donald Trump and his wife, Melania, were to visit Florida to survey the damage on Thursday.
Macron pledges relief fund
French President Emmanuel Macron was visiting the Caribbean island of Saint Barthelemy on Wednesday, a day after travelling to the French-Dutch island of Saint Martin.
France, Britain and the Netherlands have come in for criticism for the pace of relief efforts for their overseas territories ravaged by the storm.
Islanders have complained of a breakdown in law and order and widespread shortages of food, water and electricity.
Touring Saint Martin, Macron was at times jeered by people waiting for aid supplies or hoping to catch flights for France in order to escape the devastation across the island.
Macron pledged on Saint Barthelemy, population 9,000, that an emergency relief fund for victims would be operational by Monday.
Hurricane Irma caused extensive damage in the French and Dutch overseas territories as well as the British and US Virgin Islands.
British Foreign Secretary Boris Johnson was visiting the British Virgin Islands and Anguilla on Wednesday and pledged "absolute commitment" to Britons there.
His visit came as British Prime Minister Theresa May announced an extra £25 million ($33.2 million, 27.2 million euros) in aid for Caribbean territories devastated by Hurricane Irma.
A day earlier Johnson visited Bridgetown, the capital of Barbados, and met with some of the nearly 1,000 British military personnel sent to bolster relief efforts and security.
King Willem-Alexander of the Netherlands has also toured Sint Maarten, the Dutch section of the island shared with France, meeting residents on Saba island, and was set to travel to Sint Eustatius, which suffered severe damage.
(FRANCE 24 with AFP, REUTERS)