Fire investigators on Tuesday said a holiday tree ignited by a lighter caused a deadly rowhome fire in Fairmount last week that killed 12 people, including eight children. 

"We believe with certainty - so 99 to 100% confidence - that the first item ignited in this blaze was a [holiday] tree," Philadelphia Fire Commissioner Adam Thiel said. "We believe with near certainty, based on the evidence, the ignition source for this tree was a lighter that was located nearby."

Thiel detailed the "exhaustive" investigation that included looking for all possible sources of ignition and sifting through charred debris. Investigators eventually concluded that a lighter found near the tree sparked the deadly fire.

Earlier in the week, a warrant revealed that authorities were investigating if a 5-year-old child playing with a lighter ignited the deadly blaze. Thiel did not confirm the theory, but he said that a 5-year-old was the only person on the second floor where the tree and the lighter were located at the time of the fire. ... Read More: FOX29