FREMONT, Neb. – Two days after a horrific explosion at a biofuels plant left a man and two girls dead, the state fire marshal told First Alert 6 the incident started with a dust fire — and was an accident. “A preliminary investigation indicates an accidental dust explosion occurred during the manufacturing process. Additional investigation efforts are currently impeded due to the instability of the structure,” the Nebraska Fire Marshal agency said in a statement Thursday afternoon. The state has completed its initial investigation of the incident at Horizon Biofuels, which manufactures wood pellets and animal bedding. Occupational Safety and Health Administration , however, remains at the scene and must — by law — complete its investigation within six months. The Nebraska Department of Environment and Energy investigated an air quality complaint at Horizon Biofuels earlier this year. At that time, they did find wood dust beyond the property line.