An electric space heater is being blamed for sparking the horrific high-rise Bronx apartment fire that killed at least 19 people, including nine children, city officials confirmed Sunday evening.

The fire originated in a duplex on the second and third floors of the building but never extended past the unit and the hallway nearest the apartment, FDNY Commissioner Dan Nigro revealed. Smoke generated from the duplex blaze was able to filter out through an open door and spread throughout the 19-story structure, he explained.

“The door to that apartment unfortunately when the residents left was left open, it did not close by itself. The smoke spread throughout the building, thus the tremendous loss of life,” Nigro said at an evening press briefing.

The functionality of the building’s fire alarms were under investigation, but Nigro said one did alert a neighbor to the rising smoke and prompted the initial 911 call. He also said the building’s heat was working as well.

Hours earlier, the commissioner and Mayor Eric Adams revealed the devastating toll of the morning fire: a total of 63 people had suffered injuries. Thirty-two had been hospitalized with life-threatening injuries and another nine had injuries that were serious. Sadly, 19 of those victims died.

The overwhelming majority of the fire’s victims were suffering from severe smoke inhalation, Nigro said. Firefighters rushing into the building discovered victims on all nearly every floor of the building — many found experiencing cardiac and respiratory arrest.

Names and ages of the victims have not been disclosed, but fire officials confirmed that of the 19 dead, nine were children.

Elected leaders at the evening press conference explained that many of the building’s residents were Muslims from Gambia. The governor, who was on hand and met with many of the victims, said a fund would be established for the fire victims.

“This is going to be one of the worst fires we have witnessed here in modern times in the City of New York,” Adams said earlier in the day.

In Photos: Bronx Blaze, Worst NYC Fire in 30+ Years, Leaves Dozens Gravely Injured

Blown out windows and broken glass line the front of the towering apartment structure. Videos capturing the early moments of the blaze show flames spewing out from the duplex windows while smoke covered nearly the entire building.

“There were certainly people trapped in their apartments all through this building,” Nigro said, calling the smoke conditions “unprecedented.” He said it was unusual that the smoke extended the entire length of the building.

The department estimates 200 of its members responded to the 19-story residential building on East 181st Street in Fordham Heights around 11 a.m. By 1 p.m., the department had the 5-alarm fire knocked down.

Firefighters rescued countless residents of the high-rise structure by evacuating them through windows and climbing down ladders. Several of the members, the commissioner said, ran out of oxygen in the course of conducting rescues and continued operations without oxygen.

Karen Dejesus said she was making breakfast in her third floor apartment when she started to see smoke.

“We just hovered in the back room … and next thing we know we see flames coming out the back windows and stuff,” she told News 4 New York. “We smelling the smoke and the next thing we know, the whole upstairs and my apartment is black and the fire department breaking in the door to come get us. We had to climb out the window and everything.”

City officials opened a nearby school, the Angelo Patri Middle School on Webster Avenue, to residents of the building who needed assistance. The Red Cross is also offering assistance to families.

Survivors of the harrowing Bronx blaze tell shocking stories of their escape from the smoky inferno. Ida Siegal reports.

Nigro and Adams called Sunday’s disaster one of the worst fires in the city in at least three decades. More than 30 years ago, on March, 25, 1990, a fire was intentionally set at the Happy Land social club in the Bronx and 87 people were killed.

Asked about social media rumors of a resident jumping from the building to reach safety, the commissioner said someone may have slipped when attempting to get onto a fire ladder, but he had not received any reports of a jumper.

New York Gov. Kathy Hochul tweeted that she was “horrified” by the fire Sunday.

“My heart is with the loved ones of all those we’ve tragically lost, all of those impacted and with our heroic  @FDNY firefighters,” Hochul wrote. “The entire State of New York stands with New York City.“