A toddler playing with a kitchen stove started the fire that killed 12 people in a New York City apartment building.
Officials said the unattended three-year-old began to scream as the first floor room filled with smoke and flames, shortly before 7 p.m on Thursday.
Alerted, his mother grabbed him and his younger sibling but didn't shut the door as they fled to safety.
"She exited her apartment...and left the door open," Daniel Nigro, the city's fire department commissioner, told reporters at a news conference on Friday.
"So this fire quickly spread up the stairs. Fire travels up. The stairway acted like a chimney. It took the fire so quickly upstairs that people had very little time to react."
At least four children were among those killed.
Excluding the September 11 attacks, Thursday's blaze in the Bronx was New York City's deadliest since 1990. Of 14 people injured, four are fighting for their lives.
"The only thing I saw were the African people who live on the second floor. One of the kids is dead," said local resident Milica Garcia, who escaped the fire.
Residents and fire crews also had to contend with freezing temperatures.
New York City Mayor Bill de Blasio recognised concerns over the 100-year old apartment block but said there did not appear to be anything problematic with fire safety in the building.
The building is in the Belmont section of the Bronx, a primarily residential, close-knit neighbourhood known as the "Little Italy" of the borough, near Fordham University and the Bronx Zoo.
It was the deadliest fire in the city since an arsonist torched a Bronx nightclub in 1990, killing 87 people inside the venue that did not have fire exits, alarms or sprinklers, the New York Times reported.
In 2007, 10 immigrants from Mali, including nine children, died after a space heater caught fire in a Bronx building.