It’s likely that someone who didn’t belong inside a public housing complex for D.C. seniors started the massive blaze that destroyed it, federal investigators found, but because of factors that include bungled building security, they might never know how it began.

Investigators detailed numerous missteps in the management of the Arthur Capper Senior Public Housing complex before and after the Sept. 19 blaze. A recent report by the Bureau of Alcohol, Tobacco, Firearms and Explosives deemed the cause of the fire “undetermined” but said the “most likely cause of this fire was some form of human activity in the cockloft of the building,” a small attic-like space below the roof that contained electrical wiring, air conditioning units and an exhaust fan.

The ATF report indicated it remains unclear who had access to the area before the fire, due, in part, to a malfunctioning surveillance system and lax security that residents said had been a problem for months.

The report, residents said, made clear how lucky they were to escape with their lives. It also highlights mistakes that some District officials said should serve as a cautionary tale.

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Edgewood Management, which oversaw the building, is continuing “to work closely with the building owner and local and federal authorities to learn more about exactly what happened and why,” chief executive Cindy Sanquist said in a statement.

The fire burned through the roof of the complex and into the fourth floor of the building. Thick, black smoke filled the halls and billowed into the sky.

About a dozen Marines, neighbors and crews of firefighters ran into the burning building to pull residents to safety.

“I thank God that I’m living,” said Joyce Lewis, 74, who lived on the top floor and was asleep when the flames tore through the roof. “Everything burned up. Everything. And I would have, too, they hadn’t come shouting at my door.”

Everyone who lived in the building, in the 900 block of Fifth Street SE, survived — including 74-year-old Raymond Holton, who remained trapped inside his second-floor apartment for five days.

After the fire was extinguished, fire investigators “remained concerned” that someone could still be inside, according to the ATF report. The evacuation had been chaotic, but Edgewood Management assured officials that all residents had been accounted for.

Two days later, the report said, the D.C. Arson Task Force arrived with dogs to conduct a search, but they were denied entry by D.C. Fire Chief Gregory Dean in consultation with an engineer.

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D.C. Fire officials said Friday that Dean “was assured that everyone attached to the building was accounted for.” The building, part of which had collapsed, had been deemed unsafe, prompting Dean to determine that allowing a search team to enter would have risked “unnecessary danger,” department spokesman Doug Buchanan wrote in an email.

“We’ve got to be able to take that hard look in the mirror and realize what we did do right and what we did do wrong,” said D.C. Council member Charles Allen (D-Ward 6). “A lesson that we have to take from this is we can never, ever depend on a management company to tell us everyone is accounted for.”

The fire ignited above Apartment 431, a top-floor unit on the inside of the building, officials said. Fire investigators found a green garden hose near the attic entrance on the day of the blaze, in what federal officials called “clear evidence that there was some attempt by an unknown individual to fight the fire in its early stages.”

Residents who lived on the fourth floor reported regularly hearing footsteps above their units and smelling smoke from cigarettes, marijuana and other drugs, according to investigators.

Who might have gained access to the attic area that day remains a mystery.

Nonresidents routinely entered through the front door — which was staffed by security officers — a back door with a disabled alarm or the garage, former residents said.

Months earlier, a 48-year-old man was stabbed inside the building about 2 a.m. The victim and a man charged in the case lived in Hyattsville, and authorities say it’s not clear why they were in the building.

Federal investigators said the roof and attic were locked, but they weren’t hard to get into — a flathead screwdriver could be used to pry the roof door open, and screws holding the attic hatch closed were easy to loosen.

“There had been issues in the past with outside individuals gaining access into the building and ‘squatting’ in different areas,” the report said.

Security cameras had been malfunctioning for months, according to the report. Guards charged with stopping trespassers often were distracted or asleep, former residents said.

D.C. Housing Authority, which oversaw the Arthur Capper complex and works with Edgewood Management in another building at 635 Edgewood St. NE, declined to comment on alleged security lapses in the building or its relationship with the firm.

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The D.C. Council next week will hold an oversight hearing for D.C. Fire, in which “we’ll be spending a fair amount of time reviewing what we’ve learned,” Allen said.

A majority of the 132 residents who lived at the complex are being represented by D.C. law firm Arnold & Porter. Lawyers are “still evaluating options,” senior associate Lauren Daniel said.

Residents working with the firm hope pursuing legal action might help them regain lost belongings and answer nagging questions about what happened and why.

For months after the fire, residents lived in hotel rooms provided by the District government while they awaited placement in new homes. As of last month, nearly all had been put in new buildings, but their recovery is far from finished.

Dorothy Price, 71, has a new apartment, but no furniture.

James Witcher, 73, said he hasn’t been able to stop coughing.

Emily Jackson, 76, has been struggling with depression since her dog, Snoopy, died in the blaze.

Lewis wakes up in the middle of the night in a panic, convinced she can smell smoke creeping in through the window.

“I still be smelling smoke, thinking everything I had is going to burn up again — all those gifts from my children, little cards and things they made me, a picture of me and my sister and my momma. All that’s gone,” she said, crying. “And I can’t stop thinking about it.”

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