Philadelphia Fire Commissioner Lloyd Ayers announced the Fire Marshal's determination on Wednesday afternoon.
The fire occurred on Saturday, July 2 at 2:05 p.m. in a two-story home at 545 Herman Street.
With the support of neighbors and Good Samaritans, a teenage boy and girl were able to jump onto a backyard tree from the second floor and climb down to safety.
However, the neighbors couldn't get to a 4-year-old boy also inside the home.
"There was a futon stuck in the window," said Michael Vines. "So, I'm trying to pull the futon out of the way so the boy could get out, but there was so much smoke."
The child was found badly burned and unconscious on a second floor room. He was pronounced dead soon after.
The fire was extinguished in about 20 minutes.
Smoke alarms were found in the home, but they did not have batteries.