NEW YORK (AP) — Investigators sought answers Monday for why key safety features failed when fire broke out in a New York high-rise, funneling thick smoke through the tower and killing 17 people, including eight children, in the city's deadliest blaze in 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 — the only method of escape in a building too tall for fire escapes — into dark, ash-choked death traps.

Fire Commissioner Daniel Nigro said the apartment’s front door and a door on the 15th floor should have been self-closing, blunting the spread of smoke, but the doors malfunctioned and stayed fully open.

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.

Some people could not escape because of the smoke, Nigro said. Others became incapacitated as they tried to get out. Firefighters found victims on every floor, many in cardiac and respiratory arrest, Nigro said.

Limp children were given oxygen after they were carried out. Some who fled had soot-covered faces.

Firefighters continued making rescues even after their air supplies ran out, Adams said.

“Their oxygen tanks were empty, and they still pushed through the smoke,” he said.

An investigation was underway to determine 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.

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 smoke alarms, but several residents said they initially ignored them because alarms 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. “Our thoughts are with the families and friends of those who lost their lives or were injured, and we are here to support them as we recover from this horrific fire.”

A spokeswoman for the ownership group, Kelly Magee, said that 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.

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

She said 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.

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. “Would I love to see every building with sprinklers in our country? Absolutely. But it’s going to take time.”

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!’”

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.

Luis Rosa said he initially thought it was a false alarm. By the time he opened the door of his 13th-floor apartment, the smoke was so thick he couldn’t 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.”

“All we could do was wait,” he said.

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. The borough was also the scene of a deadly apartment building fire in 2017 that killed 13 people and a 2007 fire, also started by a space heater, that killed nine.

Sunday’s fire happened just days after 12 people, including eight children, were killed in a house fire in Philadelphia.