The Philadelphia fire on January 5 was the worst the city has seen in a century.

Twelve people were killed in the fire.

This includes three sisters and nine of their children.

All of the fatalities were on the third floor of the three-story row home.

Fire officials are now “nearly certain” they know the cause of the fire.

A “traumatized five-year-old child” was playing around with a lighter.

He reportedly told several people that he set the family Christmas tree on fire.

“Investigators believe that lighter was the reason the tree ignited,” officials state.

Further questions remain on why no smoke detectors were working in the building.