PHILADELPHIA (AP) — A Christmas tree fire caused the blaze that killed 12 relatives in a Philadelphia rowhome duplex, investigators confirmed Tuesday as they identified victims and announced preliminary findings. But investigators stopped short of officially concluding that a 5-year-old boy playing with a lighter was behind it.

Fire Commissioner Adam Thiel hesitated to point blame at a news conference Tuesday but said the boy was the only person on that floor. The boy, who survived the fire, told several people he was playing with the lighter when the tree caught fire, according to a search warrant issued last week.

“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 six smoke alarms inside the two-story unit were working, Thiel said, and most had apparently been removed since an inspection the city housing authority said it conducted last year. One alarm in a shared basement did work but activated late, he said.

As many as 18 people lived or stayed in the unit at various times, officials said previously.

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

Firefighters arrived within minutes at the duplex near the Philadelphia Museum of Art, but it was already too late, Thiel said. The only other survivor was a man who jumped out of a third-floor window.

“There was zero visibility,” said Thiel, noting that temperatures reached nearly 1,000 degrees at the ceiling. “Toxic smoke filling the entire building. And it’s loud in a fire.

“Rest assured, those firefighters did their level best, as our medics did their best to save those lives,” he said. “Sometimes we are too late.”

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.

The city also identified the other victims as Quientien Tate-McDonald, Destiny McDonald, Dekwan Robinson, J’Kwan Robinson, Taniesha Robinson, Tiffany Robinson, Shaniece Wayne, Natasha Wayne and Janiyah Roberts. Officials did not provide their ages.

Fires involving Christmas trees are much more likely to be fatal than other types of house fires because they become fully engulfed so quickly, according to the National Fire Protection Association, a nonprofit safety group. There are about 160 Christmas tree fires each year in the U.S., and on average, they cause two deaths, the group said.

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.

Flames damaged only a small part of that 19-story building in the Bronx, but smoke poured through the building after tenants fleeing the unit where the blaze began left the apartment door open behind them in their hurry to escape, officials said.