FEMA's Long urged communities such as Mexico Beach, where many homes were pulverized by 12 to 14 feet (3.7 to 4.3 meters) of storm surge, to rebuild to withstand future storms.
"It's OK if you want to live on the coast or on top of a mountain that sees wildfires or whatever, but you have to build to a higher standard," he said. "If we're going to rebuild, do it right."
By Friday morning the storm remnants were about 275 miles (445 km) southwest of Nantucket, Massachusetts, packing maximum sustained winds of 65 mph (100 kph).
More than 940,000 homes and businesses on the U.S. East Coast were without power and it could be weeks before power is restored to the most damaged parts of Florida.
The numbers in emergency shelters were expected to swell to 20,000 across five states by Friday, said Brad Kieserman of the American Red Cross. The Coast Guard said it rescued 129 people.
Even as rescuers searched the rubble for survivors or the dead, some Mexico Beach residents were nervously trickling back, expressing hope the place, nicknamed "the Forgotten Coast," would not change too much as it rebuilds.
"It was the perfect beach town. Not all of that commercialism," said Dottie Sinclair, 57, a nurse, adding a Subway fast-food restaurant was the only commercial chain.
"I don't think it will ever be the same," said her husband Danny, 64, who is semi-retired. "People will just pack up and leave."
