Bronx, New York - A broken electrical heater is responsible for the devastating fire in a Bronx apartment building that killed at least 19 people on Sunday, as the mayor issued an important warning to help save lives during future fires.

New York Mayor Eric Adams warned Monday that the death toll could rise from this weekend's horrific apartment building fire in the Bronx, labeled as one of the worst in modern times.

19 people, including nine children, are already dead from Sunday’s blaze, but Adams said his team fears that tragic toll could tick up as several victims remain hospitalized in critical condition.

"We believe, unfortunately, that it may," Adams said on CNN. "We pray to God they are able to pull through."

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He added in a Monday briefing that he had been in communication with President Biden, who had offered the Bronx his full support.

The five-alarm fire erupted in a 19-story apartment tower in Fordham Heights around 11 AM on Sunday after a space heater malfunctioned in the bedroom of a second-floor apartment, according to FDNY officials.

"This fire started in a bedroom in a portable electric heater," New York City Fire Commissioner Dan Nigro said in a press conference on Sunday.

In addition, he said, a door was left open after people inside had escaped, allowing deadly smoke to rapidly spread throughout the building and trapping residents in their homes.

Adams said FDNY investigators are looking into the possibility that some of the building’s self-closing doors weren’t operating properly. By law, apartment buildings in the city must have self-closing hallway doors.

Nigro said that the building may not have been up to fire code in certain respects, though he acknowledged it was built in the 1970s, when such laws were different.

Investigators are also scrutinizing reports that the building had malfunctioning fire alarms that would go off regularly for no reason, making residents ill-prepared for Sunday’s tragedy.

"This is a wake-up call for all of our buildings," Adams said.

About 200 firefighters responded to the scene, using ladders to get residents out of the building. According to Mayor Adams, some firefighters continued to search for survivors even after they had already run out of oxygen.

Videos on social media showed images of thick smoke billowing through shattered windows.

One of the building's occupants told how he heard people shouting "help, help, help" as they fled. While his children were unharmed, his neighbor's children were killed in the fire, he said, adding that he usually takes them all to school every morning.

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"What am I going to tell my kids now?" he said through tears. "Tomorrow is a school day, they're going to ask me where their friends are."

Resident Dilenny Rodriguez also described hearing children screaming, and said she herself was in her apartment when she noticed the smoke, waiting in the bedroom with her daughter until firefighters finally knocked on the door. They were then able to climb down the stairwell together.

"The bad thing was when we went down the stairs and we saw a body and dead dogs, there was nothing we could do," she said, adding that she mourned the loss of her neighbors. "Everyone had their family here in the building."

Mayor Eric Adams told reporters there's a key message to take from the tragedy.

"Close the door" when escaping from a fire behind you, he said. "If we can drill that in, we can save lives ... This painful moment can turn into a purposeful moment."

The fire ranks as the deadliest in the city since the 1990 blaze at the Bronx’s Happy Land social club, and comes just days after Philadelphia's deadliest fire in over a century.