A first-floor apartment in the Bronx building where 12 people died as fire swept through Thursday night had defective carbon monoxide and smoke detectors months ago, according to a city agency.

Authorities say the fire started on the first floor, but it’s unclear whether it was in the apartment with the faulty detectors. Reports of the defective smoke and carbon monoxide detectors remain open, and it’s unclear whether the devices had been replaced.

Mayor Bill de Blasio told CNN the apartment building is believed to be more than a 100 years old. De Blasio called the deadly fire that broke there the “worst fire tragedy in this city in at least a quarter century.”

The victims, ranging in age from 1 to 63, died on various floors as smoke and flames tore through the 29-unit apartment building.

At least four of the victims were children. Three were under the age of 10.

Details about the deadly fire — including the cause — remain unclear. Here’s what we know:

• A 1-year-old, a 2-year-old and a 7-year-old are among those killed, according to the NYPD. The age of the fourth child has not been released.

• Seven civilians and seven firefighters were injured in the fire, FDNY spokesperson said. Four injured civilians are in critical condition.

• According to the New York City Housing Preservation and Development online records, there is at least one still-open violation reported in August of this year for the building, indicating there was a defective carbon monoxide detector in a first-floor apartment, and one open violation indicating a defective smoke detector in the same apartment.

• Four people were critically injured and others were hurt, the mayor said. He added that it’s possible that “we may lose others as well.

• “This will rank as one of the worst losses of life to a fire in many, many years,” de Blasio said in a Thursday press conference.

Somber scene on frigid morning

Fire vehicles surrounded the charred apartment building Friday morning. Lights from fire engines reflected on Christmas decorations that still hang around the neighborhood. An American flag floated in the frigid air at a school across the street from the building.

The fire started on the first floor and spread quickly, said the city’s fire commissioner, Daniel Nigro. The victims died on various floors of the apartment building.

The first call about the fire came at 6:51 p.m. ET Thursday, and the New York City Fire Department (FDNY) responded within three minutes, Nigro said.

“This tragedy is, without question, historic in its magnitude,” he said. “Our hearts go out to every family who lost a loved one here and everyone fighting for their lives.”

Desperate to escape

Radio traffic from fire officials responding to the blaze paint a desperate scene.

“Bunch of calls on this one, chief,” one official said as units rushed to the Bronx.

“Baby trapped … apartment 13 is on the third floor — that’s where the baby is.”

Seconds later, the radio blared, “We’ve got people on the fire escape.”

Witnesses also shared harrowing accounts of residents scrambling onto fire escapes to get away from the growing flames.

“People were on the fire escape trying to get down on their own,” Kimberly Wilkins told CNN affiliate WCBS. “People were screaming.”

Rafael Gonzalez, another witness, said, “All I saw was just kids in the fire escape … And the flames were still going.”

Shocked residents and family members stood outside the building in frigid temperatures, waiting for news. A local school was opened to shelter those who were displaced by the fire.

Video from the scene showed one man dressed in shorts trying to help firefighters maneuver a victim on a stretcher away from the building.

A woman, who declined to share her full name with CNN affiliate WPIX, stood anxiously on the street as firefighters wheeled victims out. She said the last text she had gotten from her mother, who lives in the building, was that she was trapped in her third floor apartment.

The walk-up building contained 29 units, according to public property records. At least 12 people were rescued and are expected to survive, the mayor said Thursday night. By the time officials held a news conference around 10 p.m. ET, the fire had been put out and the building was being searched.

Deadliest New York City fire since 1990

Thursday night’s apartment fire in the Bronx is the deadliest since 1990, when 87 people died in a blaze at the Happy Land social club.

In 2007, 10 people — nine of them children — were killed in fire at a Bronx residence after a space heater cord overheated.

Friday morning Mayor de Blasio said the search for answers in Thursday night’s fire is ongoing.

“This is a terrible tragedy in the middle of the holidays. When people are trying to be with their families and have a moment of peace finally and having something like this happen to people in the Bronx is just horrible,” the myor told CNN Friday.

In addition to the reports of one apartment’s faulty smoke and carbon monoxide detectors, the city received four complaints about the building, since 2004. None of those complaints appear to be fire-related. The latest complaint entered Thursday was from the FDNY requesting a structural stability inspection after the deadly, five-alarm fire.