Tamarack, ID – An explosion in a fireplace that had been converted from propane to wood-burning fuel caused the fire that consumed a house and killed four people at Tamarack Resort in June, Idaho State Fire Marshal Knute Sandahl said Thursday.

Sandahl said that the explosion was ruled accidental.

Sometime about 2015, Sandahl said, the fireplace on the main floor of the home was converted from propane-fueled to wood-burning. During the conversion to wood, the propane line that fed the fireplace was left in place. On the night of the accident, people in the home unknowingly turned on the gas line. This allowed gas to pool in a void behind the fireplace, which exploded when it reached burning paper left in the fireplace as kindling.

The origin of the fire was determined to be the fireplace on the second floor, or main, living room of the house. When the home was originally built in 2006, that fireplace was one of four in the building fueled by propane. The cabin was then sold in 2010 and 2013. In 2015, that main floor fireplace was converted from propane to wood-burning; Sandahl declined to comment on whether the conversion was done by a licensed installer. The other three fireplaces remained propane-fueled. In May 2017, the property was foreclosed on, and ownership reverted to the previous owner. That same month, it was added to Tamarack’s pool of rentals.

It’s unclear whether the current owner of the cabin was aware of the conversion. Sandahl said no signs or instructions alerted renters to the fact that the fireplace — which still had a functional, key-operated propane valve — was meant to be used with wood fuel. Though no key was found in the rubble of the home, Sandahl said investigators believe one of the adults used a key from one of the remaining propane fireplaces to turn on the valve.

Officials declined to comment on whether the individual who converted the fireplace faces any liability in the explosion, citing “the potential of a civil case.”

In July, Valley County Coroner Scott Carver identified the victims as Erin Smith, 34; her daughter, Autumn Smith, 7; James “Jim” Harper III, 49; and James “JJ” Harper IV, 14. Smith and Harper served in the National Guard along with Smith’s husband, William “Mitch” Smith, 46, who survived the fire.

Carver said Thursday that the Smiths and the elder Harper likely died quickly in the explosion. JJ Harper’s autopsy showed soot in his lungs, indicating he had taken breaths after the explosion.

Mitch Smith was sitting in a corner of the living room, while others were trying to light a fire, Sandahl said.

Mitch Smith fled the home and was met by a neighbor who, along with Smith, entered the cabin to try to rescue the others. However, Sandahl said, they were forced from the building by the flames.