Just one year ago, New York City officials were celebrating the lowest number of fire fatalities the city had ever seen in over 100 years.

On Thursday, however, on one of the coldest and windiest nights of the year, a deadly blaze swept through a Bronx apartment building, killing at least 12 people, including a one-year-old and his mother.

It was the largest number of fatalities in a single fire in New York since 1990, when 87 people perished in an arson attack on a Bronx social club – less than a mile from the scene of last night’s deadly fire.

“We’re here at the scene of an unspeakable tragedy in the middle of the holiday season, a time when families are ­together,” said New York Mayor Bill de Blasio (D) at a press conference Thursday evening, with temperatures in the teens. “Here in the Bronx, there are families that have been torn apart.”

But while Mayor de Blasio’s words of anguish stand in stark contrast to his words of celebration just one year ago, the city has come a long way from the “Bronx is burning” stereotypes of the 1970s.

Back then, New York’s five boroughs saw upwards of 250 to 300 fatalities every year from fires. By 2000, however, the city experienced half that number. There hasn’t been more than 100 fire fatalities in one year since 2006.

In 2016, there were only 48 civilian deaths from fires, the Fire Department of New York reported – the lowest number in over 100 years of record keeping. And over the past few years, the Bronx has generally seen fewer fatalities than other boroughs. In 2015, there were 59 fatalities throughout the city, but only 3 were in the Bronx.

"Multiple fatality fires are very rare nowadays,” says Marcelo Hirschler, a fire safety expert with GBH International, a fire safety consulting firm in Mill Valley, Calif. “[And] to a large extent, fires are killing fewer people one at a time.”

Part of the reason for this, says Mr. Hirschler and other fire experts, is that the United States has become a world leader in building codes – especially when it comes to taller buildings.

New York City officials, too, point to a combination of initiatives the Fire Department has undertaken over the past decade, including increasing fire inspections around the city, improving response times to emergencies, and providing residents with smoke alarms and more fire-prevention education.

Similar efforts around the country, experts say, has helped make the risk of dying in a fire drop 21.6 percent over the past decade, according to the U.S. Fire Administration.

But where the US struggles, Hirschler says, is less about codes and more about culture. “Fires are very much a function of income, and what you find is that lower-income families tend to have more fires, largely because they tend to have stuff that’s older, stuff that’s been worn out and more likely to fail and propagate the fire.”

New York City officials said Friday that a 3-year-old boy had been playing with the kitchen stove, which then started a small fire. As it grew, the child’s mother took the boy and his 2-year-old sibling and fled the apartment, CNN reported.

But she left the door open, fire officials said, a fatal mistake that caused the fire to burst into the rest of the floor, where the stairway acted “like a chimney,” sending smoke and flames to the rest of the building.

"Close the door, close the door, close the door," said FDNY Commissioner Daniel Nigro on Friday, emphasizing one of his department's frequent messages to residents trying to escape a fire, especially in a city in which most people live stacked on top of each other in multi-unit apartment buildings.

The building also had open violations for broken smoke and carbon monoxide detectors, city officials said, though de Blasio said the violations did not appear to contribute to the fire’s fatalities.

“When it comes to public policy on clustering people – whether you are a migrant camp in Beijing, whether you are a refugee camp in Jordan, whether you’re living in Port-au-Prince, or whether you’re living in Rio de Janeiro – it all comes down to the ability of, how do you separate people, so if one person has a problem you don’t lose the whole community?” says Robert Schroeder, a former firefighter who now heads a fire and explosion analysis firm in Minneapolis.

“If people are living in common places, like that apartment complex in the Bronx, [you need to design it] so if you have a fire in one unit, you don’t take out the whole building,” he says.

Fire alarms and smoke detectors have played a huge role in reducing fire deaths in the US, Dr. Schroeder says, but he also points out that there is an unrecognized and important element in the construction and renovation of buildings: gypsum wall board.

“It is actually endothermic,” he says. “Sheet rock contains 50 percent by weight and 20 percent by volume chemically bound water, so as you get a fire in a room, the sheet rock is actually throttling it down.”

It’s another reason residents should make sure all doors are closed after fleeing a fire, experts say.

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The tragedy and loss of life in the Bronx on Thursday was “without question historic in its magnitude,” Commissioner Nigro said. “Our hearts go out to every family that lost a loved one here and everyone that’s fighting for their lives.”

The mayor, too, urged New Yorkers to keep the victims in their thoughts during the rest of the holidays. “This evening, hold your families close and keep these families here in the Bronx in your prayers,” de Blasio said.