The U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives released the final investigative report into the cause of the huge blaze at Barclay Friends Senior Living Community in West Chester, about 25 miles (40 kilometers) west of Philadelphia.
The fire likely started near a patio under an overhang at the multi-building facility, investigators said, but they could not determine the exact origin or cause because of the extensive damage. Local officials requested the federal agency's investigative help because of the fire's size and the scope of the damage.
The main valve to the internal sprinklers had been switched off in the building where the fire started, investigators said. It couldn't be determined when the valve was turned off or by whom, ATF officials said, but they did not believe the fire was set intentionally.
The ATF does not anticipate any criminal charges or any continued investigation specifically into the sprinkler valve, said Charlene Hennessy, a spokeswoman for the bureau in Philadelphia. Investigators conducted more than 300 interviews, she said.
The report did not speculate whether the lack of internal sprinklers played a role in how long it took to put out the Nov. 16, 2017, fire. The heat was so intense that one of the more than 400 firefighters battling the blaze discovered his helmet was melting.
Investigators doubted the internal sprinklers would have helped much because of where the fire started, Hennessy said.
The Barclay facility's executive director posted a letter to residents on its website Thursday.
"We are deeply distressed to have it affirmed, according to the ATF's investigation, that the main valve to the Woolman Building's sprinkler-system was believed to be closed," wrote Linda M. Sterthous. "This information confirms our worst fears, and we have taken steps to make sure this can never have again."
The flames were fanned by wind that pushed the flames up vinyl siding and onto rooftops, the report said. The report noted the facility did not have external sprinklers, which would have helped, but said they are not required under building codes. The victims' families were given access to the report's findings in advance, ATF officials said.
The lack of water, problems with sprinkler design and issues with not ensuring flame-retardant materials were used on the outside of the buildings were mentioned in lawsuits filed against the nursing home by the estates of the four residents killed.
The family of a married couple who died, Thomas Parker, 92, and Delores Parker, 89, sued the home in June, saying the couple, who both had dementia, were left to die "terrifying, gruesome, agonizing and lonely deaths."
Almost 150 residents and staff were evacuated, some in wheelchairs or rolled out on their beds into near-freezing temperatures. Dozens of neighbors also helped, wrapping residents in blankets and ferrying them to ambulances on makeshift gurneys.
The four elderly residents were unaccounted for after the evacuation and were later found dead inside the buildings. The county coroner later said they died of smoke inhalation.