A New York City apartment block fire was more deadly because an open door allowed smoke to spread, the city’s fire commissioner has said.

The blaze on the morning of 9 January in the Bronx was caused by a faulty electric heater, but fire chief Daniel Nigro said many of the victims were killed by smoke.

The heater had been in a second-storey unit but as the unit’s residents fled the fire, a door was left open, Mr Nigro said, adding that this allowed the smoke to spread.

He said the door had either been left open by residents or had failed to close automatically.

The flames damaged only a small part of the building but the smoke swept through the stairwells which, in the absence of fire escapes, meant many residents were cut off from their only means of escape.

Some were trapped in their homes, while many who did try to flee “could not escape because of the volume of the smoke”, Mr Nigro said.

Firefighters found victims on every floor, many of them in cardiac and respiratory arrest, he added.

New York City Mayor Eric Adams told Good Morning America: “There may have been a maintenance issue with this door and that is going to be part of the… ongoing investigation.”

Some 17 people, including eight children, died in what has become the city’s deadliest fire in three decades. The death toll was revised downwards by officials, who had previously reported it to be 19, including nine children.

Sixty-two people were injured and 30 of them were being treated in hospital on Monday, many in critical condition.

Large new apartment buildings in the city are required to have sprinkler systems and interior doors that close automatically to contain smoke.

It is not clear whether the same rules applied to the 120-unit Twin Parks North West building, which was constructed in 1972.

But Mr Adams confirmed the building’s doors were meant to be self-closing.

The building had smoke alarms but several residents, including Luis Rosa, said they had initially ignored them because false alarms were so common.

Mr Rosa said that, by the time he opened the door of his 13th-floor apartment, the smoke was so thick he could not see down the hallway.

“So I said, OK, we can’t run down the stairs because if we run down the stairs, we’re going to end up suffocating,” he said.

“All we could do was wait.”