A deadly house fire that killed a former New York cop, his wife and two daughters, was accidentally sparked by a cigarette flicked by the son, officials said.

Larchmont police Capt. Thomas Sullivan Sr., 48, died trying to save his family during the fast-moving blaze in Carmel on May 1.

Investigators announced on Thursday that the Putnam County fire was started accidentally by the blaze's lone survivor, Thomas Sullivan Jr., 20, after he flicked a cigarette into mulch.

The son was smoking on the front porch twice that night between 8:00pm and 10:00pm local time, slowly igniting the blaze that engulfed the house by 1:43am, officials said.

No charges will be filed against the young man.

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