Investigators sought answers Monday for why safety doors failed to close when fire broke out in a New York high-rise, allowing thick smoke to rise through the tower and kill 17 people, including eight children, in the city’s deadliest blaze in more than three decades.

A malfunctioning electric space heater apparently started the fire Sunday in the 19-story building in the Bronx, fire officials said. The flames damaged only a small part of the building, but smoke poured through the apartment’s open door and turned stairwells into dark, ash-choked death traps. The stairs were the only method of escape in a tower too tall for fire escapes.

Fire Commissioner Daniel Nigro said the apartment’s front door and a door on the 15th floor should have been self-closing and blunted the spread of smoke, but the doors stayed fully open. It was not clear if the doors failed mechanically or if they had been manually disabled. Nigro said the apartment door was not obstructed.

The heavy smoke blocked some residents from escaping and incapacitated others as they tried to flee, fire officials said. Firefighters carried out limp children and gave them oxygen and continued making rescues even after their air supplies ran out.

Glenn Corbett, a fire science professor at John Jay College in New York City, said closed doors are vital to containing fire and smoke, especially in buildings that do not have automatic sprinkler systems.

“It’s pretty remarkable that the failure of one door could lead to how many deaths we had here, but that’s the reality of it,” Corbett said. “That one door played a critical role in allowing the fire to spread and the smoke and heat to spread vertically through the building.”

Dozens of people were hospitalized, including several in critical condition. Mayor Eric Adams called it an “unspeakable tragedy” at a news conference near the scene Monday.

“This tragedy is not going to define us,” Adams said. “It is going to show our resiliency.”

Adams lowered the death toll from an initial report Sunday, saying that two fewer people were killed than originally thought. Nigro said patients were taken to seven hospitals and “there was a bit of a double count.”

The dead included children as young as 4 years old, City Council Member Oswald Feliz said.

An investigation was underway to determine exactly how the fire spread and whether anything could have been done to prevent or contain the blaze, Nigro said.

A fire department official said the space heater had been running for a “prolonged period” before the fire began. What caused it to malfunction remains under investigation, spokesman Frank Dwyer said. Fire then spread quickly to nearby furniture and bedding, Dwyer said.

Nigro said the heat was on in the building before the fire started, and the space heater was being used to supplement it.

But Stefan Beauvogui, who lived with his wife in the building for about seven years, said cold was an ongoing problem in his fourth-floor apartment. Beauvogui said he had three space heaters for the winter — for the bedrooms and the sitting room. The heating system that was supposed to warm the apartment “don’t work for nothing.” He said he had complained, but it had not been fixed.

Large, new apartment buildings are required to have sprinkler systems and interior doors that swing shut automatically to contain smoke and deprive fires of oxygen, but those rules do not apply to thousands of the city’s older buildings.

The building was equipped with self-closing doors and smoke alarms, but several residents said they initially ignored the alarms because they were so common in the 120-unit building.

Bronx Park Phase III Preservation LLC, the group that owns the building, said it was cooperating fully with the fire department and the city and working to assist residents.

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

A spokeswoman for the ownership group, Kelly Magee, said maintenance staff in July fixed the lock on the front door of the apartment in which the fire started and, while doing that repair, checked that the apartment’s self-closing door was working. No issues were reported with the door after that point, Magee said.

New York City inspectors have issued violations for problems with self-closing doors on five apartments in the building and one opening to a stairwell stretching back a dozen years, according to a database maintained by the Department of Housing Preservation and Development. The records state that all the violations were corrected.

Residents smoking in the stairwells sometimes tripped the fire alarms, and property managers had been working with them to address the problem, Magee said. She said the alarms appeared to work properly on Sunday.

The tower was required by building codes to have sprinklers only in its trash compactor and laundry room because it has concrete ceilings and floors, she said.

Camber Property Group is one of three firms in the ownership group that purchased the building in 2020 as part of $166 million purchase of eight affordable housing buildings in the borough. One of Camber’s founders, Rick Gropper, served on Adams’ transition team, advising him on housing. He contributed to a dozen politicians in the past few elections, including $400 to Adams’ campaign last year.

New York City has been slow to require sprinklers for older apartment buildings, passing laws to mandate them in high-rise office towers after 9/11 but punting in recent years on a bill that would require such measures in residential buildings.

In 2018, a city lawmaker proposed requiring automatic fire sprinklers in residential buildings 40 feet or taller by the end of 2029, but that measure never passed, and the lawmaker recently left office.

A sprinkler system set off by heat in the apartment might have saved lives, said Ronald Siarnicki, executive director of the National Fallen Firefighters Foundation.

“Most likely it would have extinguished that fire or at least held it in check and not produced the amount of toxic smoke,” said Siarnicki, adding that firefighter groups have been lobbying for stricter sprinkler requirements for years.

The building is home to many families originally from Gambia in West Africa.

Resident Karen Dejesus said she was used to hearing the fire alarm go off.

“Not until I actually saw the smoke coming in the door did I realize it was a real fire, and I began to hear people yelling, ‘Help! Help! Help!’” she said.

Dejesus, who was in her two-floor apartment with her son and 3-year-old granddaughter, immediately called family members and ran to get towels to put under the door. But smoke began coming down her stairs before the 56-year-old resident could get the towels, so the three ran to the back of the apartment.

“It was so scary,” she said. “Just the fact that we’re in a building that’s burning and you don’t know how you’re going to get out. You don’t know if the firefighters are going to get to you in time.”

Firefighters broke down her door and helped all three out the window and down a ladder to safety. Dejesus clung to her rescuer on the way down.

The fire was New York City’s deadliest since 1990, when 87 people died in an arson at the Happy Land social club, also in the Bronx. Sunday’s fire happened just days after 12 people, including eight children, were killed in a house fire in Philadelphia.