Numerous houses in Mexico Beach were reduced to naked concrete foundations or piles of debris.
The sheriff's office in Gadsden County near Tallahassee says it "can now confirm 4 storm-related fatalities following Hurricane Michael", all of which happened "in relation to or occurred during the storm".
US Army personnel used heavy equipment to push a path through debris in Mexico Beach to allow rescuers through to search for trapped residents, survivors or casualties, as Blackhawk helicopters circled overhead.
As of Thursday evening, more than 900,000 homes and businesses in the country's Southeast region had no electricity.
FEMA's Long said he's anxious the number of deaths will rise Thursday.
Brooks said several hours passed before emergency officials could reach the unincorporated area where the incident took place.
Hurricane Michael brings widespread damage and destruction to the Florida Panhandle; Mike Tobin reports.
An 11-year-old girl in Georgia also died when Michael's winds picked up a carport and dropped it through the roof of her grandparents' home. A pine tree punched a hole in their roof and he said the roar of the storm sounded like a jet engine as the winds accelerated.
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Vance Beu, 29, was staying with his mother at her home, Spring Gate Apartments, a complex of single-storey wood frame buildings where they piled up mattresses around themselves for protection.
Michael weakened as it crossed rain-soaked Georgia and moved northeast toward SC, the National Hurricane Center said.
At 11 a.m., the center of the storm was 35 miles of Charlotte, North Carolina. Emergency responders weren't able to get there in time to save her, thanks to the debris clogging the roads.
One of the carport's legs punctured the roof and hit her in the head.
"I think you're going to see it climb", FEMA Administrator Brock Long said of the death count at a news conference.
In his letter posted on the base's website, Laidlaw says crews need to clear trees from roads, fix power lines and "assess the structural integrity of our buildings" before anyone returns.
Gov. Roy Cooper in a morning briefing with reporters emphasized that the former hurricane remains a risky storm, even though most of its destructive power has been spent in Florida, Georgia and Alabama. Though most homes were still standing, no property was left undamaged. "I'm literally shocked at the scale of the destruction", storm chaser Josh Morgerman tweeted Wednesday night. So many families have lost everything. The fate of many other residents was unknown, authorities said. Entire roofs were torn away in the town of about 1,000 people, now a scene of utter devastation.
Michael, the third most powerful storm ever to strike the USA mainland, weakened into a tropical storm over SC.