Philadelphia investigators believe that last week’s fatal rowhouse fire in Fairmount, which killed 12 people, began when a 5-year-old boy set a Christmas tree aflame with a lighter and the blaze became “untenable and deadly” within minutes, Fire Commissioner Adam Thiel said Tuesday.

Speaking at a news conference alongside Mayor Jim Kenney and other city and federal officials, Thiel said investigators concluded with “99% to 100% confidence” that the Christmas tree was the first thing in the house to burn, a determination based on what he called an “exhaustive” six-day examination of nearby electrical outlets, debris, and other physical evidence.

Investigators found a lighter near the tree, and they determined through interviews that the boy was the only person on the same floor as the tree when the fire began before dawn, Thiel said. The findings essentially confirmed what the boy — who was able to flee and survived — told first responders in the immediate aftermath, according to police records obtained by The Inquirer.

One other person in the top unit of the duplex survived — a man on the third floor escaped through a window and was hospitalized with serious injuries.

Those who were killed died of smoke inhalation, according to the Medical Examiner’s Office. The city formally identified the victims Tuesday as sisters Rosalee McDonald, 33, Virginia Thomas, 30, and Quinsha White, 18, and nine of their children, ages 2 to 16.

Kenney said city officials met with surviving members of the family Tuesday to update them on the findings.

“Since that horrible day, the entire city has been mourning,” he said. “We are devastated by the loss of 12 lives.”

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

The fire broke out in the Fairmount rowhouse on the 800 block of North 23rd Street just after 6:30 a.m. on Jan. 5 and was the deadliest conflagration in Philadelphia in a generation. The day of the incident, Fire Department leadership said its preliminary investigation showed 26 people were in the duplex — which is owned and operated by the Philadelphia Housing Authority — with 18 in the upper unit and eight in the lower unit.

On Tuesday, authorities revised that and said 14 people were in the upper unit. A lawyer for the family who lived in the lower unit said five people were there at the time the fire broke out.

About 75 first responders battled the blaze, the first arriving within five minutes of the initial 911 calls. When firefighters arrived, they found the second floor, where the tree was located, filled with thick smoke, and heavy flames were pouring out the windows.

Thiel said the blaze likely spread from the tree, across the living room, and moved up an open stairwell within three minutes of ignition, sending extreme heat and toxic smoke into the third floor, where the nine children and three adults died. Temperatures inside the three-story building likely reached around 900 to 1,000 degrees, he said.

There were no working smoke detectors in the unit — five had batteries removed and one was destroyed, Thiel said. One smoke alarm in the basement sounded, but it was “late,” he said. The smoke likely rose from the second to the third floor, then filled the house and reached the basement last. The apartment was not equipped with a fire extinguisher.

» READ MORE: A step-by-step look at the Fairmount fire and how it spread

Even if smoke alarms had sounded, there was no way to escape the third floor besides going out a window — the building did not have a fire escape, so the only way to get to the front or rear door was down the staircase that was on fire, and Thiel said there was “zero visibility.”

“I’m not sure there was much of an opportunity for evacuation,” he said. He added that a full report on the fire will likely take months to complete.

Fire escapes and sprinkler systems are not required under Philadelphia building codes or PHA policy for structures the size of the rowhouse in Fairmount, city and PHA officials said. The smoke detectors that were disabled in the unit — which were installed by PHA — were an older style that used nine-volt batteries. Newer, higher-quality alarms take 10-year lithium batteries and are generally tamper-proof, Thiel said.

The family in the upper unit had lived in the four-bedroom apartment for about a decade, and 14 people were listed on the lease, PHA records show.

Kelvin Jeremiah, the chief executive of the housing authority, said there is nothing in local or federal policies that precludes intergenerational families from living together. He said the authority is conducting its own review of the incident, which he said highlights “the fundamental truth that there is in fact an affordable housing crisis in this city.”

“It remains ... a horrific, horrific tragedy,” Jeremiah said. “Our hearts ache for those who died and for the survivors. We grieve with them, and we ask all Philadelphians to keep them in your thoughts and prayers.”

Funerals for those who died have not yet been planned.