Officials pass flowers and other items left in memory of victims of Wednesday's fatal fire in the Fairmount neighborhood of Philadelphia on Thursday, Jan. 6, 2022. Officials say it's the city's deadliest single fire in at least a century.

PHILADELPHIA — Investigators in Philadelphia have ruled out other potential causes of a rowhouse blaze that killed 12 people and are left with the words of “a traumatized 5-year-old child” who said he ignited a Christmas tree while playing with a lighter, the fire commissioner said Tuesday.

Investigators are nearly certain that the tree was ignited by the nearby lighter and that the boy was the only person on the second floor of the three-story building at the time, Fire Commissioner Adam Thiel said at a news conference at which authorities released the preliminary findings of the investigation.

The boy, one of just two people in his family’s two-story apartment to survive, had told several people he was playing with the lighter, according to a search warrant. The theory involving the boy setting fire to the tree had previously been disclosed in the search warrant application last week.

The 12 people who died were all on the third floor, Thiel said. One was found alive but died at the scene.

“Investigators believe that lighter was the reason the tree ignited,” Thiel said. “We are left with the words of that 5-year-old child, that traumatized 5-year-old child, to help us understand how the lighter and the tree came together with tragic consequences.”

None of the six smoke alarms found in the unit were working, and only one was still installed. A seventh unit in the shared basement did appear to work, Thiel said.

The three-story brick duplex was owned by the Philadelphia Housing Authority, which is the city’s public housing agency and the state’s biggest landlord.

Housing officials said last week that the building, which had a separate lower unit, had 13 tamper-resistant, 10-year detectors, all of which were operational during the last inspection in May 2021.

Three sisters — Rosalee McDonald, Virginia Thomas and Quinsha White — and nine of their children died in the blaze, according to family members. The blaze was the deadliest fire in Philadelphia in more than a century.

As many as 18 people lived or stayed in the unit at various time.

Fires involving Christmas trees are much more likely to be fatal than other types of house fires, according to the National Fire Protection Association, a nonprofit safety group.

Wednesday’s blaze had been the deadliest fire in years at a U.S. residential building but was surpassed Sunday by a fire in a high-rise in New York City’s Bronx borough that killed 17 people, including several children.