Investigators are looking into the possibility a five-year-old child who was playing with a lighter set a Christmas tree on fire, sparking a conflagration that killed 12 people in a Philadelphia apartment building.
The revelation was included in a search warrant application as city and federal investigators sought to determine the cause of the city's deadliest single blaze in more than a century.
Wednesday's blaze took the lives of four adults and eight children.
Jane Roh, spokeswoman for District Attorney Larry Krasner, confirmed the contents of the search warrant, which was first reported by The Philadelphia Inquirer.
Fire officials provided few details at an afternoon news briefing, declining to say how many people escaped the blaze or speculate on a possible cause, adding the fire scene was complex.
"We will hopefully be able to provide a specific origin and cause to this fire, and to provide some answers to the loved ones and, really, to the city," said Matthew Varisco, who leads the Philadelphia branch of the federal Bureau of Alcohol, Tobacco, Firearms and Explosives.
ATF specialists and other investigators took photos and combed through the charred, three-storey brick duplex.
The building is owned by the Philadelphia Housing Authority, the city's public housing agency and the state's biggest landlord.
Fourteen people were authorised to live in the four-bedroom upper apartment that "suffered the tragedy", according to Kelvin Jeremiah, the housing authority's president and CEO, while six people were on the lease in the lower unit.
When the family upstairs became tenants in 2011, there were six people - a grandmother, her three daughters and two of their children, Jeremiah said.
He said the family grew over the next decade to add another eight children.
PHA "does not evict people because they have children", Jeremiah said.
"This was an intact family who chose to live together. We don't kick out our family members ... who might not have other suitable housing options," he said.
The fire department previously said none of the four smoke alarms in the building appeared to have been working but housing authority officials said on Thursday the building had 13 tamper-resistant, 10-year detectors in the units, all of which were operational during the last inspection in May 2021.
At least two people were hospitalised and some others managed to escape from the building, officials have said, with officials saying on Wednesday that 26 people were staying in the two apartments.
The city's fire marshal Deputy Chief Dennis Marrigan called the building a "very, very sophisticated scene. It's a very traumatic scene, and it's very complicated. It's a very complex investigation".
Officials did not release the names or ages of those killed in the blaze, which started about 6.30am on Wednesday.
Family members on Facebook have identified two of the victims as sisters Rosalee McDonald, 33, and Virginia Thomas, 30.
The siblings each had multiple children, but it is unclear if all of them were home at the time of the fire or how many of them died.
Fire officials had initially said 13 people died, seven of them children, but those figures were later updated.
Wednesday's blaze was the deadliest fire at a US residential apartment building since 2017, when 13 people died in an apartment in the Bronx neighbourhood of New York City, according to data from the National Fire Protection Association.
That fire started after a three-year-old boy was playing with stove burners.