However, there are still many unanswered questions.
The fire broke out just before 6:40 a.m. in a Philadelphia Public Housing Authority property in the 800 block of N. 23rd Street.
The house was split into two apartments, Unit A and Unit B. Sources say most, if not all, of the fatalities occurred in Unit B, which was comprised of the second and third floors.
Here's what we know so far:
CAUSE OF THE FIRE
Sources tell Action News that investigators are looking into the possibility the blaze might have been sparked after a child started a Christmas tree fire.
However, the cause of the fire remains under investigation.
The Bureau of Alcohol, Tobacco, Firearms and Explosives said Thursday morning it is bringing in the ATF's National Response team "given the magnitude and scope of the fire."
The team specializes in determining the origin and causes of fires.
The National Response Team has been part of multiple high-profile cases, including the 9/11 attack on the Pentagon and the Oklahoma City bombing in 1995.
Firefighters say the home had no working smoke detectors.
The Philadelphia Housing Authority said the smoke detectors were working during an inspection in the spring of 2021.
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Action News has learned there were seven smoke detectors and three carbon monoxide detectors in one of the apartments, and six smoke detectors with three carbon monoxide detectors in the other as of the last inspection in the spring of 2021.
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It was not known why those smoke detectors didn't work.
The duplex is a Philadelphia Housing Authority property. In a statement, PHA President and CEO Kelvin A. Jeremiah confirmed the property was last inspected in the spring, and all smoke detectors were "operating properly at that time."
NUMBER OF OCCUPANTS
Eight people lived in Unit A, which was on the first floor of the building, and another 18 people lived in Unit B, which was comprised of the second and third floors of the home.
However, the Philadelphia Housing Authority said they were only aware of 14 residents in Unit B when they did their last occupancy recertification in October.
Deputy Fire Commissioner Craig Murphy noted that it was a large number of people to be occupying a duplex, but a spokesperson for Philadelphia's Department of Licenses and Inspections said the city does not limit the number of family members who can stay in a single unit.
PHA spokesperson Kirk Dorn says the family moved into Unit B roughly a decade ago, and at the time six people occupied it. Over the years the family expanded with young children.
Dorn says the family never requested to be moved to a larger unit. He says due to the pandemic and demand for low-income housing, PHA could not move the family to a five-bedroom home, but it's unclear why the family wasn't split up into two smaller homes.
Family members identified some of the victims who lost their lives in this horrific fire.
Qaadira and Jacuita Purifoy say they lost their three sisters and their eight children. They identified one of them as 32-year-old Virginia Thomas.
A second sister, identified as Rosalee McDonald, also perished in the blaze, along with her six children: three boys and three girls.
Isaiah Brown said several of the victims were his cousins, but he loved them like brothers and sisters.
"One was 16, one was like 10... and seven. You know, they were babies - babies, man. Young children. Didn't even get a chance at life," Brown said.
Amidst the pain and the grief, a solemn procession of police vans carried the bodies of the victims to the medical examiner's office Wednesday night.
Van after van passed through the intersection of 23rd & Parrish streets as relatives and friends looked on in deep sorrow trying to make sense of the tragedy.
Eight people were able to get out of the house, officials said.
Two people were taken to the hospital. One went to Children's Hospital of Philadelphia and one went to Temple University Hospital. There was no immediate word on their conditions.
EMERGENCY RESPONSE TIMELINE
City officials say the first 911 calls about the fire were answered right away, and firefighters were on the scene within minutes.
A press release, they said that police radio received 36 calls to 911 between 6:36 a.m. and 6:39 a.m. on Wednesday concerning the fire on the 800 block of N. 23rd Street.
"The first call was received at 6:36:28 and was answered immediately," the city said.
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Officials said the call taker retrieved the information regarding the location of the fire and transferred the call to fire communications at 6:37:12 a.m.
The call was answered on two rings, the city said.
Firefighters were then sent on their way two minutes after that first call came in.
The first firefighters arrived four minutes from the time of the first call.
"Fire personnel were dispatched and en route at 6:38:27. The first Philadelphia Fire Department company arrived on scene at 6:40 a.m.," the city said.
DEADLIEST PHILLY FIRES
This is the deadliest single fire in Philadelphia in at least a century.
Before this, the city's deadliest fire was in October 1901. A fire in an eight-story building on the 1200 block of Market Street killed 22 people.
A family of nine, including seven children, died in a fire back in 1967 on Parrish Street.
Then in May 1985, 11 people died on the 6200 block of Osage Avenue when police dropped explosives on the roof of a house where members of the group MOVE were holed up.
COMMUNITY SHOCKED, HORRIFIED
Neighbor Bill Richards who lived on the block for 24 years says he heard a woman yell, "Oh my God! Oh my God!"
He then heard fire trucks and came outside.
"It's very upsetting," Richards said. "I just can't wrap myself around it."
Richards spoke about a young man he has watched grow up who lives in the building.
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"It's a great neighborhood - a varied neighborhood. There are houses that have single families, multi-families, there are apartments, a lot of renters but there are single families. I've been here for 24 years. I love the neighborhood," Richards said.
Lindsay Hull told Action News she came upon the scene at 7 a.m. She saw stretchers.
She said her friend lives in the building next to the rowhome.
"That's a house that has a lot of kids," Hull said. "It's sad."
First lady Jill Biden, who along with President Joe Biden has deep ties to the Philadelphia area, tweeted, "My heart is with the families and loved ones of the victims of the tragic fire in Philadelphia."
My heart is with the families and loved ones of the victims of the tragic fire in Philadelphia.— Jill Biden (@FLOTUS) January 5, 2022
People could be seen sobbing uncontrollably outside the burned-out home.
Neighbors told Action News they woke up to smell the smoke and could hear the sirens. They looked outside to see the street lined with fire trucks.
"You come out. You think it's not close. You think it's never as close as it is," neighbor Michelle said.
She told Action News about another fire that happened before Thanksgiving on the same block.
"Twice," Michelle said. "It's horrible."
"Without a doubt one of the most tragic days in our city's history," Mayor Jim Kenney said at a news conference. "Losing so many kids is just devastating... Keep these babies in your prayers."
The Associated Press contributed to this report