There is no question as to what started the fire on December 28, 2017, at 2363 Prospect Avenue in New York’s Bronx neighborhood: The blaze began when a 3-year-old boy was toying with the stove burners in his family’s apartment. What officials now must determine is whether the outcome may have been less devastating if safety features in the building were different. It will be a difficult question to answer, with the interior of the building extensively charred and some apartments burnt completely, but one that may help prevent similar structure fire incidents in the future.

Stairwell “Acted Like a Chimney” in New York Structure Fire

The apartment where the fire originated is on the first floor of the Bronx apartment building. It began while an unidentified 3-year-old boy was playing with the burners on the kitchen stove, which New York fire personnel say they now know he had done before. The boy’s mother, also unidentified, was in the bathroom when she heard his screams and found flames quickly spreading through the kitchen. She gathered both her son and another 2-year-old child and fled the apartment through the front door, which remained open behind the family.

The fire leaped from the apartment into the stairwell on a hunt for oxygen, and turned the five-story stairwell into something of a chimney, spreading the flames faster and preventing residents from escaping down the stairs.

“The stairway acted like a chimney,” New York City Fire Commissioner Daniel Nigro said in a statement. “People had very little time to react…they couldn’t get back down the stairs. Those that tried, a few of them perished.”

Firefighters Found Hydrant Frozen at Prospect Avenue Fire

Oxygen flooded the building as residents opened windows to get air or to access the fire escape, further feeding the fire. Calls began to flood the fire department at 6:51 p.m. In one, a caller described a baby trapped inside Apartment 13 on the third floor of the building.

Only three minutes after the first call reached the fire department, firefighters arrived at 2363 Prospect Avenue to battle the blaze. Their first obstacle was instant—the hydrant in front of the building was frozen from the icy temperatures. The firefighters still entered the building, where they immediately discovered three victims deceased in the entrance hall. Ambulances were called, and firefighters pushed on in an effort to aid other residents. Another hydrant, on nearby 187th street, was located and used to fill hoses.

Rescuers found five deceased victims inside the apartment building, and seven more individuals, some of them children, were pronounced dead at area hospitals. A thirteenth victim—27-year-old Holt Francis—was in a coma for a week before being declared brain dead and taken off life support, according to a family member. Francis’ wife, two daughters, and niece also died in the fire.

It is the deadliest fire in New York City since 1990, when Happy Land Social Club, also in the Bronx, caught fire, killing 87 people.

Fire Safety Advocates Want Improved Stairwells in N.Y. After Bronx Building Fire

The chimney-effect of the stairwell is unsurprising given the age of the apartment building, which was built in 1916 and is constructed of plaster and brick. Safety experts say that advances in fire-proof technology have given way to firewalls that may hold off a blaze for hours instead of minutes, allowing those inside time to escape. Those walls were not in place in the Bronx building fire, but some say they should have been.

New Jersey recently passed a law requiring that older buildings fireproof their stairwells, and officials believe the move had a noticeable impact in the state.

“That retrofit law has actually saved lives over the years,” Glenn Corbett, a fire safety expert with John Jay College, said in an interview with NBC 4 New York. “Because we don’t have as much of an issue with open stairwells in larger apartment buildings in New Jersey.”

New York does not have a similar law, with older buildings instead being exempt from fireproofing their stairwells.

The Bronx building fire demonstrated just how dangerous those older stairwells can be, with residents attempting to escape from upper floors through the stairwell.

“They had come down the stair case to flee their apartments,” FDNY Deputy Commissioner Frank Gribbon told The Post. “They were coming down into the throat of the fire.”

Building Was Required to Have Self-Closing Doors, But Officials Are Unsure If It Did

Another safety element that officials will examine in the investigation into the Bronx building fire is the door that was left open between the apartment where the fire started and the hallway.

The apartment building, being more than three units, was required to have self-closing fireproof doors, which means that when the mother and her two children fled, the door should have shut behind them, locking the fire in and buying more time for other residents to escape. What investigators will try to determine is whether the door did not function properly, was propped open by the apartment residents, or was not the required type of door.

Matthew Creegan is a spokesperson for the NYC Department of Housing Preservation and Development and told NBC 4 New York that it will take some work to figure out.

“Because of damage caused by the fire, it’s unclear if the unit had such a door and/or if there was an obstruction that prevented it from closing as the family fled,” Creegan said via email.

The apartment building is owned by D&A Equities. Ronn Torossian, a spokesperson for the company, said that they are working with the city. Torossian did not comment on whether the building had self-closing fireproof doors.

At Least One Safety Violation Was Open Against Apartment Building

An official statement on the doors in the apartment building may take some time, but there is at least one open violation against the building that is already known.

New York City Department of Housing Preservation and Development records show that the building received a report of a violation in August. The report indicated that there was a defective carbon monoxide detector and a faulty smoke detector in a first-floor apartment. Investigators have not said whether either were a factor in the December 28 fire or if the apartment where the fire started was the same apartment that had the faulty equipment.

Small fire safety devices and adherence to building codes can mean the difference between deadly fires and survivable ones. Many buildings that are the scenes of such tragedies, like the Oakland Ghost Ship warehouse fire, have received prior citations for safety violations.