The trail of destruction was at least half a mile wide and overwhelmed rural Lee County's coroners' office, forcing it to call in help from the state.
"The devastation is incredible," Lee County Sheriff Jay Jones said.
Drones flying overheard equipped with heat-seeking devices had scanned the area for survivors, but the dangerous conditions halted the search late Sunday, Sheriff Jones said. Rescuers planned to resume the search at daylight Monday.
The Sunday tornado was part of a powerful storm system that also slashed its way across parts of Georgia, South Carolina and Florida.
Levi Baker, who lives near the hard-hit area in Alabama, took a chain saw to help clear a path for ambulances and other first-responder vehicles. He said some houses were demolished and trees were uprooted or snapped in half. One house was swept off its foundation and was sitting in the middle of the road.
"It was just destruction," Baker said. "There were mobile homes gone. Frames on the other side of the road."
Jones said the twister traveled straight down a county road in the rural community of Beauregard reducing homes to slabs.
The National Weather Service confirmed late Sunday a tornado with at least an F3 rating caused the destruction in Alabama. Although the statement did not give exact wind estimates, F3 storms typically are gauged at wind speeds of between 158-206 mph (254-331 kph).
After nightfall Sunday, the rain had stopped and pieces of metal debris and tree branches littered roadways in Beauregard. Two sheriff's vehicles blocked reporters and others from reaching the worst-hit area. Power appeared to be out in many places.
In a tweet late Sunday, President Donald Trump said: "To the great people of Alabama and surrounding areas: Please be careful and safe. Tornadoes and storms were truly violent and more could be coming. To the families and friends of the victims, and to the injured, God bless you all!"
Rita Smith, spokeswoman for the Lee County Emergency Management Agency, said about 150 first responders had quickly jumped in to help search the debris after the storm struck in Beauregard. At least one trained canine could be seen with search crews as numerous ambulances and emergency vehicles, lights flashing, converged on the area.
No deaths had been reported Sunday evening from storm-damaged Alabama counties other than Lee County, said Gregory Robinson, spokesman for the Alabama Emergency Management Agency. But he said crews were still surveying damage in several counties in the southwestern part of the state.
Numerous tornado warnings were posted across parts of Alabama, Georgia, Florida and South Carolina on Sunday afternoon as the storm system raced across the region. Weather officials said they confirmed other tornadoes around the region by radar alone and would send teams out Monday to assess those and other storms.
In rural Talbotton, Georgia, about 80 miles (130 kilometers) south of Atlanta, a handful of people were injured by either powerful straight-line winds or a tornado that destroyed several mobile homes and damaged other buildings, said Leigh Ann Erenheim, director of the Talbot County Emergency Management Agency.
News footage showed smashed buildings with rooftops blown away, cars overturned and debris everywhere. Trees all around had been snapped bare of branches.
"The last check I had was between six and eight injuries," Erenheim said in a phone interview. "From what I understand it was minor injuries, though one fellow did say his leg might be broken."
She said searches of damaged homes and structures had turned up no serious injuries or deaths there.
Henry Wilson of the Peach County Emergency Management Agency near Macon in central Georgia said a barn had been destroyed and trees and power poles had been snapped, leaving many in the area without power.
Authorities in southwest Georgia were searching door-to-door in darkened neighborhoods after a possible tornado touched down in the rural city of Cairo, about 33 miles (53 kilometers) north of Tallahassee, Florida, on Sunday evening. There were no immediate reports of serious injuries.
Authorities said a tornado was confirmed by radar in the Florida Panhandle late Sunday afternoon. A portion of Interstate 10 on the Panhandle was blocked in one direction for a time in Walton County in the aftermath, said Don Harrigan, a meteorologist for the National Weather Service in Tallahassee.