According to New York City Fire Commissioner Dan Nigro, the Bronx apartment fire that killed 19 people, including nine children, and injured dozens of others on Sunday morning was sparked by a defective electric space heater.

According to him, the late-morning fire quickly spread because a door in the two-level second-floor flat where the incident started was either left open or did not close automatically when residents evacuated.

"The marshals have determined through physical evidence, through firsthand accounts from the residents, that this fire started in a bedroom in a portable electric heater," Nigro said at an evening news conference.

He explained that the heater was used to boost the building's heat, which he maintained was operational at the time. The effectiveness of fire alarms, on the other hand, was being investigated.

The fire spread to five alarms, and at least 30 people were sent to the hospital. According to Nigro, a second open entrance allowed deadly smoke to spread throughout the structure.

Firefighters and inhabitants were rapidly overcome by dangerous quantities of smoke, with firefighters running out of air while performing rescues in the 19-story, 120-unit building, according to Nigro.

"They ran out of air, many of our members, and they continued working to get as many people out as they could," he said.

Many of the residents who died or who were taken to hospitals were likely to have been affected by smoke, he said. "I think some of them could not escape because of the volume of smoke," he said.

According to the fire department, over 200 firemen were dispatched to the blaze on East 181 Street.

Authorities were called to the location about 11 a.m. and were able to put out the fire by 1 p.m., according to NBC New York.

When asked about reports that inhabitants leaped out of windows, Nigro replied that some "may have fallen from the building" while attempting to get onto ladders set up by firemen.

He claimed he had not heard of anyone jumping before the firefighters came.