Investigators have finished collecting physical evidence at the site of the rowhouse fire in Fairmount that killed 12 people, and officials expect to release the preliminary findings of their review as early as this week.

Engineers and fire protection experts from the Philadelphia Fire Marshal’s Office and the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives cleared the scene Monday, according to ATF officials, who have partnered with local authorities to probe the cause of the deadly blaze.

Management of the scene was turned over to the city’s Department of Licenses and Inspections, a Fire Department spokesperson said. Officials with the Philadelphia Housing Authority, which owns and operates the three-story duplex, were also on scene securing the property along the 800 block of N. 23rd St.

Residents of the lower unit, who escaped the fire with minor injuries, returned to scope out what remained of their home, which was heavily damaged by smoke and water, though some framed pictures still hung on the walls. The fire broke out on the second floor and spread to the third, where a dozen people from the same family — three adults and nine children — died.

Officials have yet to release an official cause of the blaze. A 5-year-old who lived in the apartment that caught on fire and survived told investigators he accidentally lit a Christmas tree as he was playing with a lighter, according to police records obtained by The Inquirer.

» READ MORE: Remembering those lost in the Fairmount fire

The city has not yet identified the victims, and a spokesperson for the Medical Examiner’s Office said it won’t release the causes of death until the Fire Department’s investigation concludes. Fire officials have not said how long that investigation could last.

Relatives of those who died said they include sisters Rosalee McDonald, 33, Virginia Thomas, 30, and Quinsha White, 18, as well as nine of their children, ranging in age from 2 to 16.

Surviving family members have requested privacy as they process the loss, and funeral services have not yet been planned. One adult from the family, Howard Robinson, escaped the fire by climbing out a third-story window and remains hospitalized at Temple University Hospital in fair condition, hospital officials said.

Members of the family lived in the apartment for about a decade, and 14 people were listed on the lease for the four-bedroom apartment, PHA records show. There were six working smoke detectors in the unit at the time of its most recent inspection last spring, according to the agency. But First Deputy Fire Commissioner Craig Murphy said Wednesday that at least some of those alarms did not sound.

The unit did not have a fire extinguisher or a fire escape, neither of which are required under Philadelphia building codes or PHA policy, according to city and PHA officials.

» READ MORE: The fatal Fairmount fire was a once-in-a-generation tragedy. Here’s how it unfolded.

Authority workers Monday were clearing debris from the building. They were breaking down equipment investigators used to sift through evidence and bagging an accumulation of charred children’s clothing from the sidewalk. One worker swept away a pile of soot that had collected around a broken toy car.

All the windows of the house were blown out, and the inside of the second and third floors appeared gutted. A stray piece of crime scene tape hung on an entrance to the backyard, where seven children’s bikes were perched on the fence.

Officials at the scene preserved a collection of stuffed animals and prayer candles that friends and strangers have left at the house over the last few days. Among the mourners were Barry and Karen Reese, who traveled from Northeast Philadelphia Monday to quietly drop off 12 tiny stuffed bears. They didn’t know the family, but have 16 grandchildren themselves and felt a pull to memorialize those who died.

The fire, they said, made them think harder about the safety of their own family. Barry Reese said the couple purchased a fire extinguisher for their daughter, who lives with her family in a third-floor apartment in North Philadelphia, not far from Fairmount. They planned to make sure she knows how to use it.

“We all need a plan,” Karen Reese said. “God forbid.”

Inquirer news researcher Ryan Briggs contributed to this article.