Glenn Allen, a spokesman for Georgia Insurance and Safety Fire Commissioner John Oxendine, identified the victims as Paul McCorkle and his son Jason.
Allen said a preliminary investigation indicates Paul McCorkle was using a bulldozer about 10:45 a.m. to grade property where his two sons, Jason and Jon, lived off Stagecoach Road when he accidentally struck an 8-inch Dixie Pipeline liquid propane gas line two feet underground.
The explosion did not happen immediately, and McCorkle got off the bulldozer and drove home to nearby Belle Meade subdivision, where he called 911, said Gary Nicholson, the special agent in charge of the Georgia Bureau of Investigation's Thomson office.
While he was gone, Jason McCorkle came onto the property and was apparently killed instantly when the gas line exploded.
Officials from at least five agencies are investigating: the GBI; the county sheriff's department, coroner's office and Fire/Rescue Services; and the State Fire Marshal's Office.
"We're treating it as a tragic accident," Nicholson said. He said his agency was asked to assist by Sheriff Logan Marshall.
The gas line, which is about 30 years old, extends from Texas to North Carolina.
Paul McCorkle, a first-term county commissioner, was discharged Monday night from Joseph M. Still Burn Center at Doctors Hospital, spokeswoman Anne Cordeiro said. His injuries were described as "frostbite burns," by Fire/Rescue Services Chief Bruce Tanner.
About 20 acres of woodlands burned, as did a doublewide mobile home that was the home of Jon McCorkle, Tanner said. A shed, a Jeep and the bulldozer also burned.
On Monday afternoon, officials said the fire had spread toward Interstate 20 and caused traffic delays. Later, they said that was an unconnected woods fire.
Tanner said firefighters were not trying to stop the blaze at the rupture because there was still gas in the pipeline.
"When the fire goes out, then you've got a problem because you've got raw gas that could ignite backfires," Tanner said. "If the gas were just coming out of that line and had not ignited, it would find low places until it found an ignition source."
When the line has purged, the fire will stop on its own, he said, but that might take 24 hours -- until as late as this afternoon.
Fire officials chose not to evacuate the area and said nearby residents were not in danger.
As many as 50 cars lined Stagecoach Road on Monday afternoon as Thomson residents gathered. Some held their hands over their mouths in shock as they stared at the billowing, black clouds.
Matt Williams said he heard the explosion while he was in a nearby Waffle House and arrived at the scene about the same time as firefighters.
"It looked like a bomb went off," he said. "You don't expect anything like this in a small town like Thomson. You hear things like that in big cities, but not around here.
"It sounded like bottle rockets going off in the parking lot," Williams said. "I turned around and said 'Who in the world is shooting off bottle rockets at Waffle House?' Then I saw the mushroom cloud go up."
He said he knows the McCorkle family and "I feel empty inside."
About 50 firefighters from McDuffie, Warren, Wrens, Martinez/Columbia County, Wilkes and Richmond counties battled the fire for several hours Monday.
McDuffie Mirror reporter Billy Hobbs contributed to this article.