New York City Mayor Eric Adams, just over a week into the job, confirmed 19 people had died from the blaze that broke out around 11 am in the imposing Twin Parks North West building which provided affordable housing units.
Earlier on Sunday, officials said 32 people had been hospitalised with life-threatening injuries and some 60 people were injured in total.
"It's a tragedy beyond measure," Adams wrote on Twitter. "Join me in praying for those we lost, especially the 9 innocent young lives that were cut short."
In a press conference on Sunday afternoon, Adams said "it appears as though this stemmed from a space heater."
The fire itself started in an apartment that spanned the second and third floors of the building, and only made it to the hall, officials said.
But smoke still spread to every floor of the building, likely because the door to the apartment was left open, and victims suffered from significant smoke inhalation, the city's fire department commissioner Daniel Nigro told reporters at a news briefing.
"Members found victims on every floor in stairwells and were taking them out in cardiac and respiratory arrest," he said.
Fire marshals had determined through physical evidence and accounts from residents the fire started in a portable electric heater in the apartment's bedroom, Nigro said. He added that the heat had been on in the apartment building and that the portable heater had been supplementing that heating.
The catastrophe was likely to stir questions on safety standards in low-income city housing. This was the second major deadly fire in a residential complex in the US this week after twelve people, including eight children, were killed early on Wednesday when flames swept through a public housing apartment building in Philadelphia.
US Representative Ritchie Torres, a Democrat whose district includes the New York building, told MSNBC that affordable housing developments such as the Bronx one pose safety risks to residents. "When we allow our affordable housing developments to be plagued by decades of disinvestment, we are putting lives at risk," he said.
Adams said many of the residents were from the small west African country of Gambia.
The building did not have external fire escapes, and residents were meant to evacuate through interior stairways, Nigro said. "I think some of them could not escape because of the volume of smoke," he said.
Some 200 firefighters helped put out the blaze, and some ran out of oxygen in their tanks but pushed through anyway to rescue people from the building, Adams said.
"I really want to thank them for putting their lives on the line to save lives," Adams said.
A Reuters photographer at the scene on Sunday saw emergency responders performing CPR on at least eight people in front of the building. Firefighters with hose lines were working to push smoke out of the building, and one of them was seen breaking a window on an upper floor to release the fumes.
Nigro said he believed there were 120 apartments in the building. "There's a very large number of people right now who need a place to stay," he said.
A NYC emergency management official said everyone who needed housing would be registered and would be placed in hotels for an "extended period" until it was safe to return to the building.
The building is owned by a joint venture, Bronx Park Phase III Preservation LLC, made up of three firms. "We are devastated by the unimaginable loss of life caused by this profound tragedy," a spokesperson for the joint venture said.
Bronx Park Phase III Preservation LLC did not immediately respond to requests for information on the building's fire safety record and inspection history, affordable housing units, and management. The building was built in 1972, officials said.
Affordable Housing Online, a website that shares information on public housing, reported the building has "a certain number of units" set aside for lower-income households.
That same website listed Reliant Realty Services as the building's property manager. Reliant Realty Services did not immediately respond to a request for confirmation it was the building manager or information on the building's fire safety history.