At least 17 people have died in the biggest storm to hit Florida since the 1800s.
Rescuers say they expect the death toll from Hurricane Michael to increase as they comb through devastation left by the "monster storm".
Winds of up to 155mph uprooted trees, brought down power lines and reduced homes to splinters, lifting structures from their foundations and carrying them inland.
"Very few people live to tell what it's like to experience a storm surge," Brock Long, head of the Federal Emergency Management Agency (FEMA), said from Mexico Beach, a town flooded with a rush of 4.27m of seawater in the storm.
"I have reasons to believe we still haven't got into some of the hardest hit areas."
One elderly man is known to have died in the town where the storm made landfall, his body found hundreds of yards from his home.
Some counts have indicated that 285 people in Mexico Beach remained in their homes during the storm, but rescuers say nearly 200 people have escaped with their lives.
After making landfall in Florida on Wednesday afternoon, Michael's fierce winds and heavy rain struck Georgia, where it was downgraded to a tropical storm, and later reached the Carolinas.
North and South Carolina are both still recovering from Hurricane Florence, which left dozens dead and caused billions of dollars in damage last month.
Of those known to have died in the most recent hurricane, eight have been in Florida, five in Virginia, one in Georgia and three in North Carolina.
More than 375,000 people up and down the Gulf Coast were ordered or urged to evacuate as Michael closed in, but it moved so fast that people did not have much time to prepare.
Red Cross officials said 7,800 people were in 100 shelters across Florida, Georgia and Alabama. They said it was still not clear how many people stayed put and would need rescuing.
Though the hurricane has moved on, the scale of the recovery effort is only now beginning to become clear.
Hundreds of thousands of people remain without electricity and in some areas cellphone coverage is just 30%.
President Donald Trump has declared an emergency in Alabama and ordered federal assistance.
More from Florida
Florida governor Rick Scott said officials do "not know enough" about those who stayed behind, but said the storm had left the state "like a war zone".
"We are not completely done. We are still getting down there," he said.