As of early on Saturday, state officials were reporting that at least 18 have been killed in Florida, Georgia, North Carolina and Virginia.
Rescue teams, hampered by power and telephone outages, were going door-to-door and using cadaver dogs, drones and heavy equipment to hunt for people in the rubble in Mexico Beach and other Florida coastal communities, such as Port St. Joe and Panama City.
The Houston-based volunteer search-and-rescue network CrowdSource Rescue said its teams were trying to find about 2,100 people either reported missing or stranded and in need of help in Florida, co-founder Matthew Marchetti said.
Social media websites were crowded with messages from those trying to reach missing families in Florida's Bay and Gulf Counties.
Michael crashed ashore near Mexico Beach on Wednesday afternoon as one of the most powerful storms in U.S. history, with winds of up to 155 mph (250 kph). It pushed a wall of seawater inland, causing widespread flooding.
The tropical storm, which grew in less than two days into a Category 4 hurricane on the five-step Saffir-Simpson scale, tore apart entire neighbourhoods in the Panhandle, reducing homes to naked concrete foundations or piles of wood and siding.