Investigators don't know what happened to the vapor barrier or an associated fitting, though they believe it was left unsecured after routine maintenance, a spokesman for the Jackson County Sheriff's Office said Thursday.
Workers will inspect all 62 propane-fueled patrol cars to make sure they are safe, said Chief Deputy David Cochran.
"This is definitely an isolated incident," said John Franklin, senior engineer for propane system provider American Alternative Fuel. "The disengagement of the system's safety features is something that all safety training moving forward will address emphatically.
"The good news is that the operator of the vehicle sustained only minor injuries, that the problem doesn't lie with the equipment or installation," Franklin said.
The fire broke out about 6:30 a.m. Monday while Deputy Gary Cox drove along Dry Pond Road north of Jefferson on his way to a call. He lit a cigarette and the interior of the cruiser caught fire.
Cox escaped with first-degree burns to his face and arms. He was treated at Athens Regional Medical Center and released later that day. He has not returned to work, Cochran said.
Over the last four days, engineers and propane system installers have inspected the burned car to find what caused the leak.
When converting a police car, contractors install a bullet-resistant propane tank in the trunk, then connect it to a component that uses heat from the car engine to vaporize the liquid propane and inject the gaseous fuel into the engine cylinders.
The vapor barrier, which was not in place on Cox's car, is a nonporous hood that fits over the propane tank's nozzles and valves, and vents any fumes outside of the car, according to Cochran.
Someone failed to replace it properly during maintenance or when they refilled the tank, and consequently propane fumes slowly began to leak into the car. There also was a fitting on the tank that had not been replaced properly after maintenance.
Both the sheriff's office and Force 911, the Pendergrass company that installed the system, perform maintenance on the cars so investigators have no idea who removed the vapor barrier, Cochran said.
"We don't how the vapor cover got off of those tanks," he said. "But we are now inspecting all of our vehicles to make sure the barriers are there and that we don't have the same problem on any of the other tanks."
Monday's fire is the first problem the sheriff's office has had with the cars since they started to convert their fleet to run on both propane and gasoline. The office discontinued the use of the propane systems after the fire and will not start using them again until all the cars have been inspected.
Deputies, who prefer propane-fueled cars for their fast acceleration, were hesitant to get back into the vehicles until they knew exactly what happened in Cox's car, Cochran said.
The department also will change its maintenance regimen to make sure the cars are safe. Administrators also have barred deputies from smoking in their patrol cruisers, which was not against the sheriff's policies until now.