Teams made up of hundreds of volunteers with the Houston-based CrowdSource Rescue organization were searching for more than 1,135 people in Florida who lost contact with friends and family, Matthew Marchetti, co-founder of Houston-based CrowdSource Rescue.
Most of those missing are from Panama City and many are elderly, disabled, impoverished, or live alone, Marchetti said.
Florida officials have not given a number for how many people are considered to be missing. Those who are missing may be with relatives and friends, and not necessarily presumed dead,Presstv reported .
Debris, downed trees and power lines have hampered access to stranded people, but CrowdSource said a number of its missing person reports resulted from widespread phone and power outages.
The death toll includes 17 in Florida, one in Georgia, three in North Carolina and six in Virginia, according to a Reuters tally of official reports. Officials said medical examiners were determining whether another four deaths in Florida were due to the storm.
Michael slammed into the northwest coastal strip of Florida last Wednesday with top sustained winds of 155 miles per hour (250 km/h), unleashing a surge of seawater that demolished homes.
About 35,000 Floridians have called Federal Emergency Management Agency seeking help since and the agency has already approved $1 million in assistance for people in those 12 counties, spokesman Ruben Brown in Tallahassee said.
FEMA has distributed about 4.5 million meals, more than 5 million liters of water and 9 million infant-and-toddler kits, he said.
In Mexico Beach, which took a direct hit, the number of people missing dropped to three on Tuesday, said Rex Putnal, a city councilor. A day earlier, it was more than 30. The town of 1,200 residents had reported two fatalities as of Monday.
Nearly 163,000 homes and businesses remained without power in the US Southeast, with residents of battered coastal towns forced to cook on fires and barbecue grills.
At least 80 percent of customers in three mainly rural Panhandle counties were without electricity on Tuesday.
Officials said it could be weeks before power returns to some.
Countless others in the region's back country have struggled for days without running water or sanitation, awaiting help from authorities. Some have been camping in tents with the belongings they were able to salvage.
The state government is distributing ice, water and about 3 million ready-to-eat meals, Governor Rick Scott's office said.
Marchetti said the search has been hindered by spotty cell phone coverage in the devastated area, though authorities are making progress in restoring communications.
Many residents have also expressed frustration at the slow pace of recovery of wireless networks.
Federal Communications Commission Chairman Ajit Pai on Tuesday called for wireless carriers to waive bills for customers affected by the storm.