(CNN) -- A 3-year-old boy's screams alerted his mother that a fire had erupted in their first-floor apartment in the Bronx on Thursday night. The boy had been playing with the burners on the kitchen stove, New York City fire officials said Friday.
When the mother fled the burning apartment with the boy and his 2-year-old sibling, she made a fatal mistake -- she left the apartment door open.
The tragic result was the deaths of at least 12 people, including five children. being killed as fire and smoke spread throughout the building. It was the deadliest fire in New York City in more than 25 years.
The apartment's stairway acted "like a chimney" as the fire burst from the apartment, feeding the flames and allowing them to spread throughout the building, FDNY Commissioner Daniel Nigro said Friday.
The boy in the first-floor apartment had a history of playing with the stove burners, Nigro said. The fire commissioner reminded all New Yorkers of his department's frequent messages emphasizing the crucial step residents must take in the event of a fire.
"Close the door, close the door, close the door," Nigro said.
The fire commissioner said he's unsure whether all the smoke detectors were working in the building.
Records from a city housing agency show one of the apartments on the building's first floor reported defective carbon monoxide and smoke detectors months ago. It's unclear if those issues were resolved and whether they played any part in Thursday's fire.
Seven civilians and seven firefighters were injured in the fire, according to the fire department. Four injured civilians are in critical condition.
Desperate to escape
Fire vehicles surrounded the charred apartment building Friday morning. Lights from fire engines reflected on Christmas decorations that still hang around the neighborhood. An American flag floated in the frigid air at a school across the street from the building.
The fire started on the first floor and spread quickly, Nigro said earlier. The victims died on various floors of the apartment building.
The first call about the fire came at 6:51 p.m. ET Thursday, and the fire department responded within three minutes, Nigro said.
Radio traffic from fire officials responding to the blaze paint a desperate scene.
"Bunch of calls on this one, chief," one official said as units rushed to the Bronx.
"Baby trapped ... apartment 13 is on the third floor -- that's where the baby is."
Seconds later, the radio blared, "We've got people on the fire escape."
Witnesses also shared harrowing accounts of residents scrambling onto fire escapes to get away from the growing flames.
"People were on the fire escape trying to get down on their own," Kimberly Wilkins told CNN affiliate WCBS. "People were screaming."
Rafael Gonzalez, another witness, said, "All I saw was just kids in the fire escape ... And the flames were still going."
Shocked residents and family members stood outside the building in frigid temperatures, waiting for news. A local school was opened to shelter those who were displaced by the fire.
Video from the scene showed one man dressed in shorts trying to help firefighters maneuver a victim on a stretcher away from the building.
A woman, who declined to share her full name with CNN affiliate WPIX, stood anxiously on the street as firefighters wheeled victims out. She said the last text she had gotten from her mother, who lives in the building, was that she was trapped in her third floor apartment.
The walk-up building contained 29 units, according to public property records. At least 12 people were rescued and are expected to survive, the mayor said Thursday night. By the time officials held a news conference around 10 p.m. ET, the fire had been put out and the building was being searched.
Deadliest New York City fire since 1990
Thursday night's apartment fire in the Bronx is the deadliest since 1990, when 87 people died in a blaze at the Happy Land social club.
In 2007, 10 people -- nine of them children -- were killed in fire at a Bronx residence after a space heater cord overheated.
Friday morning Mayor de Blasio said the search for answers in Thursday night's fire is ongoing.
"This is a terrible tragedy in the middle of the holidays. When people are trying to be with their families and have a moment of peace finally and having something like this happen to people in the Bronx is just horrible," the myor told CNN Friday.
In addition to the reports of one apartment's faulty smoke and carbon monoxide detectors, the city received four complaints about the building, since 2004. None of those complaints appear to be fire-related. The latest complaint entered Thursday was from the FDNY requesting a structural stability inspection after the deadly, five-alarm fire.
