(CNN) -- Investigators have ruled out foul play as a factor in a bizarre fire that killed a Tennessee father and his 12-year-old son on Sunday.
Preliminary autopsy results released this week showed the pair, of Huron, died of carbon monoxide poisoning and thermal burning, authorities told CNN on Wednesday.
At issue was an accidental firearm discharge, which investigators said triggered the fire as well as the explosions that ensued, but it remains unclear what else contributed.
Henderson County Sheriff Brian Duke said Terry York, 48, was showing his loaded AR-15 to a family friend in the back room of his wooden mobile home when it misfired and a small fire was ignited.
"It was a bizarre misfire that I hadn't seen for more than 20 years," Duke said.
The fire didn't start out as severe but soon triggered explosions "one right after another" in minutes, according to one neighbor's account made to CNN affiliate WBBJ. Henderson County Fire Chief Lynn Murphy said it took firefighters more than four hours to contain the fire. The entire back room was burned down while the rest of the house saw little damage.
The victim's 12-year-old son died when trying to douse the flames, Duke said. The father was pronounced dead at the scene while the son died en route to the hospital.
Kate Abernathy, a spokeswoman for the state's Bomb and Arson Investigation unit, said there was black gunpowder found inside the room that the victim possibly used to reload his weapon, but it wasn't immediately known what role it played in the incident.
At least one other family member also suffered burns from attempts to enter the back room. No firefighters or neighbors were hurt, investigators said.