Thomas Sullivan Jr., center, was the lone survivor of a May 1 fire. His parents, Donna and Thomas Sr., and his sisters, Meaghan and Mairead, were killed.Putnam County Executive

The cause of a deadly house fire which almost wiped out an entire family was the cigarette butt of the sole surviving son, police said Thursday.

Carmel police Chief Michael Johnson said the cause of the deadly fire was the result of a month long investigation, the Associated Press reports

The likely cause was a cigarette butt dropped by Thomas Sullivan Jr (20) into the into the mulch bed at the foot of the porch of the family’s home on April 30. The butt caused the mulch to ignite as the fire smoldered for some time, before it spread to a plastic trellis and the wooden porch, and then engulfed the home in a deadly blaze.

The fire ripped through the suburban home in Carmel, NY  killing Thomas Sullivan Sr. (48), a Larchmont police captain and former NYPD cop; his wife, Donna and their daughters Meaghan (18), and Mairead (13). According to the coroner they died from smoke and carbon monoxide inhalation.

Johnson said the father’s heroic actions saved his son’s life. The police captain allegedly woke his son and told him to escape before he tried to rescue the rest of family.

Investigators said the son was informed about the findings of the investigation on Wednesday night.

"He's taking this very hard," Lt. Brian Karst told the AP. "He's going through the grieving process." Johnson called for compassion and support.’

"For someone who was in shock he was very honest and forthright with us," he said.

A family friend said the 20-year-old is now living with his aunt.

According to Dan Tompkins, captain of the Putnam County Fire Investigation Team, four weeks of dry weather and a windy night were all contributing factors to a "perfect storm" of conditions.