New York City’s deadliest fire in more than three decades, killing 19 people including nine children on Sunday at a Bronx apartment building, was caused by a faulty space heater, officials said.

Fire Commissioner Daniel Nigro said the blaze “started in a malfunctioning electric space heater” in an apartment unit spanning the second and third floors of the 19-story building. The door of the apartment was left open, allowing smoke to quickly spread throughout the building, he said.

Some residents, trapped in their apartments, broke windows for air and stuffed wet towels along the bottom of their doors. One man rescued by firefighters said he’d become numb to fire alarms because of frequent false alarms.

Some residents “could not escape because of the volume of smoke,” Nigro said.

Stefan Ringel, a senior adviser to Mayor Eric Adams, said the children killed were 16 years old or younger. Many of the building’s residents were originally from the West African nation of Gambia, Adams said, and there was a large Muslim community.