A fast-moving fire in a Philadelphia row house that killed 12 people last week, including eight children, likely started after a Christmas tree was lit ablaze with a lighter, fire officials said Tuesday.

The intense fire started Wednesday morning on the second story of a duplex unit, and all fatalities occurred in the same unit, city officials said.

A lighter was found near the Christmas tree on the second floor, and there appear to have been no other possible ignition sources based on the preliminary investigation, according to Philadelphia Fire Chief Adam Thiel.

“We believe with certainty, with 99 to 100% confidence, that the first item ignited in this blaze was a Christmas tree,” Thiel said during a press briefing Tuesday. “We believe with near certainty, based on the evidence, that the ignition source for that tree was a lighter that was located nearby.”

Only two people in the unit survived the fire, including a 5-year-old child, who fire officials believe was the only person on the second floor at the time the fire started, 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, because we have disproved any other theories,” Thiel said.

The fire department’s findings come after the Philadelphia Inquirer first reported last week that a 5-year-old reportedly told fire investigators that he was playing with a lighter when the Christmas tree ignited.

Thiel said Tuesday that investigators did not find anything to “disprove” the hypothesis that the 5-year-old lit the tree with the lighter. “That does not prove it, because we’re talking about a 5-year-old child,” he added.

The blaze was very fast-moving and difficult to fight, with high heat, toxic smoke and no visibility. Within three minutes, it was “untenable and deadly,” the fire chief said.

The fire has been classified as incendiary, meaning investigators don’t believe it occurred accidentally, he said.

“We believe there was some type of human intervention to bring the ignition source to the first item ignited,” Thiel said, a belief based on the “less-certain” words of the surviving child. Other agencies may classify the fire as accidental, he noted.

“I certainly understand the desire for simple, clear-cut answers. This is not a simple, clear-cut incident or investigation,” he said.

There were six fire alarms in the unit where the fire started, though none were operational, according to Thiel. Four were in drawers and inoperable, one was on the ceiling but did not have a battery, and another was on the floor of a bedroom without a battery.

A fire alarm in the basement, which was shared with another unit in the building, was operational and went off. But it was too late to warn the occupants, most of whom were on the third floor, Thiel said.

It will take several months for the department to complete a full investigation.

There were 14 occupants at the time of the fire, city officials said. Two people, including the child, were critically injured and hospitalized.

Authorities have not yet identified those killed, though city officials said a family, including three sisters and their children, lived in the apartment.

The building was operated by the Philadelphia Housing Authority. The agency is reviewing the incident and will “carefully explore all options with regard to how we can continue to ensure the safety of our residents,” Kelvin Jeremiah, director of the Philadelphia Housing Authority, said Tuesday.