In a statement Saturday, the city of Eastland called it a "tragic turn" in the death of firefighter Gregory Simmons.
Eastland officials earlier said the 51-year-old died Friday after flames overcame his fire truck and he and other firefighters jumped out.
Justice of the Peace James King said Friday that Simmons apparently was overcome by smoke and fell in a ditch.
The Texas Department of Public Safety is now leading the investigation.
State Trooper Phillip "Sparky" Dean said Saturday authorities don't yet know if Simmons died in a crash of the fire truck, or if he was fatally injured after getting out of the vehicle.