A blaze tore through a rowhouse in Philadelphia’s Fairmount section on Wednesday morning and killed 12 people, including eight children — a tragedy that officials are calling one of the worst in fires in recent memory.

The investigation carried into the evening, as fire officials and other investigators searched for clues as to how the blaze began at the building on the 800 block of North 23rd Street, where more than two dozen people were reportedly living in tight quarters.

Here’s what we know — and what we don’t — as of Wednesday evening.

» READ MORE: Fairmount house fire that killed 12 is one of the nation’s deadliest in decades

What we know

There were at least 26 people living in the three-story, multifamily rowhouse

  • The 2,300-square-foot building — owned by the Philadelphia Housing Authority since 1967 — had long been subdivided into two units, sometimes referred to as a duplex.

  • The building contained two, four-bedroom units, which split parts of the second floor, according to PHA. Fire officials said 18 people were in the upper unit and six below at the time of the fire.

A “heavy fire” broke out in the kitchen of the second-story apartment before sunrise Wednesday, officials said. Firefighters arrived within minutes.

  • Police dispatchers received three dozen 911 calls reporting the fire between 6:36 a.m. and 6:39 a.m., according to Philadelphia Police Cpl. Jasmine Reilly.

  • The first truck arrived on scene at 6:40 a.m.

  • Deputy fire commissioner Craig Murphy said the flames spread rapidly up the stairwell and through the third floor: “Nothing was slowing that fire down.”

The fire alarms in the house were not working when the blaze broke out, officials said, and the building had no fire escape on the upper floors.

  • Philadelphia Housing Authority president Kelvin Jeremiah said in a statement the property was last inspected in May and that smoke detectors were operating properly at that time. Murphy, the deputy fire commissioner, said four were not operational Wednesday.

  • The Department of Licenses and Inspections does not require escape structures for upper floors — even in split-unit or multifamily homes of this size.

  • Neighbors living in similar split-unit rowhouses nearby said they had raised concerns about the lack of evacuation options on the upper floors in recent years.

» READ MORE: Where to donate to help victims of the Fairmount fire

Eight people escaped the building on their own. Two others were rescued by firefighters.

  • Two people, including one child, were taken to area hospitals in critical condition. City officials later said the child did not survive.

The victims ranged in age from young children to adults in their 30s, according to survivors and neighbors. Some names are known at this time.

  • Virginia Thomas, Rosalee McDonald, and their children lived on one of the upper floors, said Aneisha Thomas, who was grieving the loss of her first cousins .

  • “When I go visit Philly, it’s going to be a void,” Thomas said. “It’s going to be a blank stare when I visit because of how I can’t go visit them.”

  • Children ages 7, 10, and 16 were among those lost, said Isaiah Brown, 18, who was grieving his cousins.

What we don’t know

How the fire started remains unknown

  • Fire officials said the investigation into what sparked the blaze would continue through the night, but could not provide a timeline by end of day.

  • Police were seen still pulling bodies and other debris from the building after sundown.

  • Fire marshals’ investigations can take days to provide a full account of a blaze.

How people escaped — and what happened inside — after the fire broke out

  • More information will become available as investigators speak with family members.

  • Of the eight people who evacuated and the two rescued by firefighters, it’s not clear where they were living in the house and how they managed to escape.

» READ MORE: Two sisters and their children are among the victims of the Fairmount fire

The victims have not all been identified

  • Family members and friends identified the names of a few victims, but the full scope of the dead isn’t known.

  • Family and friends of people who lived in the house that caught fire gathered at at 22nd and Parrish Streets on Wednesday morning to wait for more information.

The stories of the many people living there

  • According to L&I, city code does not include occupancy limits on family members under the same roof. PHA officials said they perform an annual check on the number of occupants at each of its buildings.

  • The housing authority told The Inquirer that 15 people were living in Unit B of the building as of last year.

  • The family had moved in a decade ago, but was never placed in a larger home even as the family expanded, PHA spokesperson Kirk Dorn told the news channel.

» READ MORE: ‘Only babies’: After Fairmount fire, Philadelphia mourns an unthinkable loss

It’s not clear if anything criminal happened

Staff writers Anna Orso, Ximena Conde, Rodrigo Torrejón, Ryan Briggs, Aubrey Whelan, Mensah M. Dean and Rob Tornoe contributed.