Sources tell Action News that investigators cleared the scene of the fire on Monday and briefed top Philadelphia officials on their findings.
The January 5 fire tore through a row house in the 800 Block of North 23rd Street in Philadelphia's Fairmount section. Eight children and four adults were killed.
Family members identified one of the victims as 32-year-old Virginia Thomas. A second sister, who was identified as Rosalee McDonald, died in the fire along with her six children: three boys and three girls.
Investigators are looking into whether a 5-year-old playing with a lighter near a Christmas tree sparked the deadly blaze.
No official cause has been determined.
SEE ALSO: What we know about the fire in Philadelphia's Fairmount section that killed 8 children, 4 adults
The Philadelphia Housing Authority says both units of the fire-ravaged duplex had working smoke detectors, all powered by 10-year lithium batteries, when housing officials inspected the property last May.
But firefighters say those smoke detectors were not working.
The Fairmount fire was the city's deadliest in more than 100 years.