Nine children were among those who died when a space heater malfunctioned in a Bronx apartment building, city officials said.

Nineteen people, including nine children, were killed on Sunday when an apartment fire started by a malfunctioning space heater sent smoke billowing through a Bronx high-rise, officials said, in the deadliest fire New York City had seen in more than three decades.

An additional 44 people were injured, 13 of them critically, after the occupants of the third-floor apartment where the fire started fled without closing the door behind them, the fire commissioner, Daniel A. Nigro, said at a news conference at the scene.

“Smoke spread throughout the building, thus the tremendous loss of life and other people fighting for their lives,” he said.

The smoke from the fire spread to the top of the 19-story building, darkening hallways and stairwells and shocking residents who had heard the fire alarms but did not immediately react because they had grown accustomed to frequent alarms in the building. Firefighters found victims on every floor and worked to rescue them even as their own oxygen tanks ran low, Commissioner Nigro said.

The fire’s toll was the worst in the city since 87 people died in an intentionally set fire at a Bronx nightclub in 1990 and was an early test for the city’s new mayor, Eric Adams. “The numbers are horrific,” Mr. Adams said at the first of two news conferences at the site.

Thirty people remained in the hospital on Sunday evening, Mr. Adams said. He urged all of the injured and displaced victims to seek help and assured those who may be undocumented that their information would not be passed along to federal immigration authorities.

Gov. Kathy Hochul, who spoke at the second news conference, said that she would include funding to assist victims with the cost of housing and burials in her budget proposal next week. She described holding a mother who had lost her entire family in the fire.

“Tonight is a night of tragedy and pain, and tomorrow we begin to rebuild,” she said.

A city official, who spoke on the condition of anonymity because the fire was still under investigation, said fire marshals believe the space heater had been running for several days uninterrupted. The residents were using the heater to supplement the building’s heat, which was on, officials said.

Apartment doors left open during fires have featured in some of the city’s worst blazes, including a Bronx fire in 2017 that left 13 people dead. The fire was started by a young boy playing with the stove in his family’s first-floor apartment and quickly tore through the building.

The building where Sunday’s deadly fire occurred is in Fordham Heights in the West Bronx. Built in 1972, it has no fire escapes, like most modern high-rises, and residents must rely on the stairwells in the event of an emergency.

Roughly 200 firefighters responded to the scene after a fire broke out at a 120-unit apartment building. Officials said it was caused by a space heater, sparking one of New York City’s worst fires in recent memory.

The building, like the surrounding neighborhood, is home to working-class families of African American, African and Hispanic descent, some of whom use federal Section 8 vouchers to help pay the rent. Residents described the building as a melting pot of races, religions and languages. Several said that the sound of the fire alarm was so common that people learned to ignore it.

Ms. Campbell, who lives on the third floor, said that the fire alarms in the building go off five or six times a day. When they do, she said, “I roll my eyes.”

The tower is owned by three investors — LIHC Investment Group, Belveron Partners and Camber Property Group — that purchased it with seven other rent-regulated buildings in the borough for $166 million in early 2020. One of Camber’s co-founders, Rick Gropper, is a housing adviser to Mr. Adams.

A spokeswoman for the building’s owners said the smoke alarms were sometimes tripped by people smoking in the stairwells. But she said they were unaware of any problems with the devices and that the alarms had sounded properly during Sunday’s fire.

“We are devastated by the unimaginable loss of life caused by this profound tragedy,” the property owners said in a statement.

Officials said the fire was the deadliest since the fire at the Happy Land nightclub in 1990 in the Bronx, located not far from the site of Sunday’s fire. The club, which operated illegally, had no sprinklers, and several exits were blocked off with roll-down security shutters.

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