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.