SALEM -- A spark from a pressure switch of a home water system is believed to have ignited a propane explosion in the basement of a Salem home on July 13 that killed six people and landed five others in the hospital.
Two independent propane systems served the home on Route 29, although one of the systems had been inactive since March 2010, according to a report issued by State Police late Thursday.
Both tanks were located outside the home and each had a capacity of 100 gallons. The active tank, which served a water heater in the basement and a stove on the first floor, contained about 72.14 gallons when weighed, two days after the incident.
The inactive tank, which was last used to serve a former cooking stove in March 2010, contained about 24.52 gallons when weighed on July 15.
The pressure switch was located on the pump of the home's water system in the basement. When water pressure drops, a mechanism kicks in to increase the water pressure and it tends to give off a spark.
Because propane is heavier than air, it typically gathers in the basement, where many potential sources of ignition exist.
According to the report, investigators determined that there was "a reported event involving the discharge of propane" in the basement just prior to the explosion, although due to the magnitude of the blast the source of the propane discharge cannot be readily identified, according to authorities.
The explosion claimed the lives of Darrell Durham, 20; his 2-month-old daughter, Niah Lynn Durham; Lawrence Berg II, 19; Tammy Palmer, 41; Clarissa Lyn Porlier, 19; and Robert Sanford, 16.
Alicia Berg remains hospitalized at Albany Medical Center. Her medical condition has been upgraded from critical to fair.
Chelsea Wilcox, who is in the burn unit of the Westchester Medical Center in Valhalla, was listed in fair condition on Thursday.
Three others were treated and released from area hospitals after suffering injuries in the blast.
State Police Capt. Steven James said authorities were still conducting interviews late Thursday and would not comment on whether criminal charges were expected.