Carmel police Chief Michael Johnson said a monthlong investigation found the likely cause was a cigarette butt or ash that Thomas Sullivan Jr. dropped into the mulch bed at the foot of the porch on the evening on April 30.
After igniting in the mulch, the fire smoldered for a few hours, then spread to a plastic trellis and the wooden porch before racing through the home in Carmel, 60 miles north of New York City, Johnson said.
The blaze killed Thomas Sullivan Sr., a Larchmont police captain; his wife, Donna; and their daughters Meaghan, 18, and Mairead, 13. The coroner said they died from breathing smoke and carbon monoxide.
Johnson said the father's "heroic actions" had saved his son. He said the police captain woke his son and sent him out of the house before apparently trying to rescue the rest of the family.
Police said the 20-year-old son was told about the investigators' conclusion Wednesday night.
"He's taking this very hard," Lt. Brian Karst said. "He's going through the grieving process." Johnson called for compassion and support.
Putnam County Executive Mary Ellen Odell, a family friend, said Sullivan is now living with an aunt.
"He'll count on the support of his family, he'll count on his faith and he'll count on the support of his community," she said.
Karst said Sullivan cooperated with police from the start and acknowledged he had been smoking on the porch.
"For someone who was in shock he was very honest and forthright with us," he said.
Odell said that under family rules he could smoke but only outside the house.
Johnson said investigators had eliminated the possibility of an intentional fire - they had looked closely at Capt. Sullivan's contacts - and the likelihood that natural gas or electricity was involved.
When they traced the point of origin to the foot of the front porch, a cigarette ember was determined to be the probable cause.
Dan Tompkins, captain of the Putnam County Fire Investigation Team, said four weeks of dry weather and a windy night contributed to a "perfect storm" of conditions.
Tompkins said that although the house complied with building codes, its modern, light construction could have contributed to the speed of the fire once it got inside.
He said investigators were never able to find out if the eight hard-wired smoke detectors in the home were working. Karst said the surviving son doesn't remember hearing an alarm.
Asked if the tragedy had taught a lesson, Tompkins said, "Don't drop your cigarette butts in mulch." Officials wouldn't say building codes should be changed, though Odell said she would be meeting soon with municipal officials to discuss the findings of the investigation. She said codes are set by each town and village in Putnam.
Thomas Sullivan Sr. was a former New York City police officer assigned to the Bronx who left for the suburbs 20 years ago, saying he felt he could make a bigger difference in a smaller community.
The family bought the home in 2000 for $388,000.
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