NEW YORK >> Doctors raced today to save survivors of New York City’s deadliest fire in three decades as authorities began investigating how thick smoke could billow through a high-rise, trapping many families inside and killing 17 people, including eight children.

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

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

Adams lowered the death toll, saying that two fewer people were killed than originally thought. Fire Commissioner Daniel 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, said City Council Member Oswald Feliz.

Investigators determined that a malfunctioning electric space heater, plugged in on a cold morning, started the fire in the 19-story building.

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.

Adams said the building had self-closing doors and that investigators were looking into whether a door malfunctioned.

“There may have been a maintenance issue with this door. And that is going to be part of the … ongoing investigation,” the mayor told ABC’s “Good Morning America.”

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.

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.

“So many of us were used to hearing that fire alarm go off, it was like second nature to us,” resident Karen Dejesus said. “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.

Hassane Badr told The New York Times that two of his siblings, both children, were killed and that a 25-year-old cousin remained unaccounted for. Badr, 28, waited at Jacobi Medical Center for news about his 12-year-old brother, who was suffering from serious smoke inhalation. A 5-year-old sister was at another hospital.

“I’m thinking like I’m dreaming, this is not true. You hear people crying, my goodness,” Badr told the newspaper. “To be honest, I’m not believing it right now.”

Badr’s family, 11 people from Mali, lived in a three-bedroom apartment on the third floor.

Mahamadou Toure struggled to put his grief into words outside the hospital emergency room where his 5-year-old daughter and the girl’s teenage brother died, according to the Daily News.

“Right now my heart is very …,” Toure trailed off while speaking to the New York Daily News. “It’s OK. I give it to God.”

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.