What are some of the most important lessons from last night’s deadly fire in a Bronx apartment building – the deadliest in New York City in over 25 years?

At a news briefing Friday morning at the site of the fire Commissioner Daniel Nigro shared the findings of fire marshals on the cause of the fire and strongly urged people to take proper fire safety precautions to prevent similar tragedies in the future.

Among those precautions are being careful not to leave children unattended.

The fire began with a 3-year-old child playing with the burners on a kitchen stove of a first floor-apartment in a five-story building in the "Little Italy" neighborhood of the Bronx.

“Fire marshals have been here all night trying to determine the cause and they have determined it," he said.

The child who started the fire was unattended by parents at the time.

After hearing the child scream, the child’s mother ran out of the home with the child and another 2-year old without closing the door behind her.

"You've seen the ads — close the door, close the door, close the door," New York's fire commissioner said Friday. "The results if you don't [close the door] are what happened here last night," said Commissioner Nigro.

"The stairway acted like a chimney. It took the fire so quickly up the stairs that people had very little time to react."

A total of twelve people have died while four people were injured critically from the tragic fire, the victims of which included young children.

New York City Mayor Bill de Blasio said the fire was "an unspeakable tragedy" at the site of the fire late Thursday night.

"In the middle of the holiday season, a time when families are together, tonight here in the Bronx there are families that have been torn apart," he said.