By CBS3 Staff

PHILADELPHIA (CBS) — Investigators are “near certain” a Christmas tree ignited by a lighter found nearby originated a tragic fire in Philadelphia’s Fairmount neighborhood last week. Twelve people, including nine children, were killed in the blaze that ripped through two Philadelphia Housing Authority units in the morning of Jan. 5.

Philadelphia Fire Chief Adam Thiel says the lit Christmas tree has been determined the initial ignition source after “exhausting investigation of the scene” and based on the testimony of a 5-year-old child.

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An extensive search of the scene led investigators with the ATF and Fire Marshal to rule out any other ignition sources.

The Christmas tree was located in the second-floor unit and Thiel says the 5-year-old was the only surviving person rescued from the second-floor.

“The words of a 5-year-old child who is essentially one of two survivors of this tragedy and really the sole survivor, the only one who was located, based on our interviews, the only person who was on the second floor which is where the tree and lighter were located at the time when we believe the fire was ignited. We are left with the words of the 5-year-old child, that traumatized child, to help us understand how the lighter and tree came together with tragic consequences because we have disproved any other theories, hypotheses based on the incredible level of expertise we brought, including ATF’s national response team and all the other partners who contributed to this investigation,” Thiel said.

Last week CBS3 learned the fire may have been started by a 5-year-old playing with a lighter near the Christmas tree, but Thiel says you can’t rely solely on the testimony of a child.

“We are almost certain that the Christmas tree located on the second floor was the item that was first ignited. We are near certain that what ignited that fire was a lighter and how the lighter came into contact with the Christmas tree, you’ve all seen and heard reported the words of a 5-year-old,” Thiel said. “We’re left with the words the only surviving person who was on the second floor at the time we believe the fire started was a 5-year-old. We are not going to get into details of the words of a 5-year-old.”

Investigators could not find anything to disprove the words of the child, but Thiel says this does not prove the child started the fire because “we are talking about the words of a 5-year-old child.”

“Since that horrible day, the entire city has been mourning and devastated by the loss of 12 lives,” Mayor Jim Kenney said.

According to the Philadelphia Department of Public Health’s Medical Examiner’s Office, the 12 people who perished in the fire died of smoke inhalation. The three adults and nine children killed were Dekwan Robinson, Destiny McDonald, Janiyah Roberts, J’Kwan Robinson, Natasha Wayne, Quientien Tate-McDonald, Quinsha White, Rosalee McDonald, Shaniece Wayne, Taniesha Robinson, Tiffany Robinson and Virginia Thomas.

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“On behalf of the entire Health Department, I want to express our deepest condolences to those who have been affected by this fire,” Health Commissioner Dr. Cheryl Bettigole said. “We mourn for each of the young mothers and children lost, everything they should have been able to become and to experience, and the tremendous loss experienced by their families, their communities, and our city in this terrible tragedy.”

Thiel says the fire has been ruled incendiary, which means officials don’t believe it occurred accidentally.

“We believe there was some type of human intervention to bring the ignition source to the first item ignited,” he said. “Again, you’ve heard me say that’s based on without certainty but the less certain words of a traumatized 5-year-old. We don’t have any hypothesis. We don’t have anything really to the contrary. What we know for sure is the tree, the next thing we know pretty sure, near certainty, is the lighter and again we’re left with the words of this 5-year-old.”

The fast-moving blaze quickly rose from the second floor consuming the three-story rowhome. Investigators say the bulk of the victims were found in bedrooms on the third floor.

Fire officials discovered a total of seven smoke detectors were found. Three were found in a kitchen drawer, another in a bedroom drawer, one on the floor and one was on a bedroom ceiling.

All of them had the 9-volt batteries removed and were inoperable.

One smoke detector was discovered in the basement and did activate, but it was too late to alert the fire victims on other floors.

Philadelphia fire officials are again reminding people to make sure they have an escape plan in place in the event of a fire and to make sure they have working smoke detectors.

They say they are continuing to install free smoke detectors throughout the city. In 2021, they distributed 1,100.

The City of Brotherly Love has come together to help those impacted by this tragedy. Click here to find out how you can donate to the victims.

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CBS3’s Natasha Brown and Kerri Corrado contributed to this report.