The house fire in Philadelphia killed 12 people including nine children, but only one of seven smoke alarms functioned at the home.
Twelve people, eight of them children, died in a fire when a five-year-old boy who survived started alight a Christmas tree and destroyed a home, authorities said.
The child was a witness and gave a version of events but he was unsure who opened fire.
Philadelphia Fire Commissioner Adam Thiel said a child believed to be the main suspect of starting the fire, and one of just two survivors, was the only person in the first floor of the building when the fire broke out.
Firefighters battled the blaze inside the three-storey semi-detached house, which had been split into two apartments, at about 6.40am on Wednesday, officials said.
Acting Deputy Fire Services Inspector Craig Murphy said that the “heavy fire” which was moving quickly, quickly engulfed the kitchen area of another second floor flat and then entered the third floor through an open staircase.
There ‘nothing slowing that fire from moving,’, Philadelphia deputy fire commissioner Craig Murphy told reporters at a press conference Thursday as the Metropolitan Police Police and the Philadelphia ATF branch assisted with the probe.
The building is owned by the state-financed Philadelphia Housing Authority, which is the fourth main housing authority in the United States.
