NEW YORK (CBSNewYork) — Close the door – that is the message from FDNY Commissioner Daniel Nigro to everyone, in the event of a fire.

As CBS2’s Dave Carlin reported, Nigro said that simple act could mean the difference between life and death.

A blaze that left 12 people dead in the Belmont section of the Bronx Thursday night was the city’s deadliest fire in more than 25 years. It grew fatal fast because of one major factor, according to Nigro.

A mother inside the apartment where the fire started wisely got herself and her kids out immediately. But in doing so, she forgot to do something vital.

“She exited her apartment with her 2-year-old and 3-year-old, and left the door open,” Nigro said.

Nigro said closing that door could have saved many of the 12 lives that were lost.

“Fire travels up. The stairway acted like a chimney,” Nigro explained. “It took the fire so quickly upstairs that people had very little time to react.”

A little more than a year ago at a 33-story tower at 515 W. 59th St. on the West Side of Manhattan, a fire injured two dozen people. A tenant in a third-floor apartment also fled without shutting the door when lit candles started the fire.

Local residents say even though many people know apartment and hallway doors should remain closed, they still do the wrong thing. They say often panic sets in, and that’s why the doors are left open.

Residents say they do it by mistake.

“It’s not intentional,” a woman named Nicole said. “You’re just panicked, so I think more drills should be done.”

“An open door just accelerates or accentuates a bad situation,” said Thomas Goodwin, Education Director at Steamfitters Local 638.

Goodwin, a fire safety educator, said more buildings need sprinkler systems to take away that human error.

He also said every tenant needs to check and recheck that detectors work properly.

“Have fire alarms. Change your batteries,” Goodwin said. “There’s no doubt early prevention and fire sprinklers would save a lot of lives.”

“You’ve seen the ads. You’ve seen the ads,” Nigro said. “Close the door. Close the door. Close the door.”

Nigro said he cannot repeat that often enough — so as to ensure that next time, the fire losses are not so unprecedented, so grave and so heartbreaking.

Fire safety educators recommend having a fire extinguisher inside your home and running fire drills with your entire family.

Starting at 9 a.m. Saturday, the FDNY will share safety information at the Church of St. Martin of Tours on Crotona Avenue.