The building on Gates Avenue erupted in flames just after 5:50 a.m. Monday and quickly collapsed like a pancake, fire officials said.
One firefighter sustained a minor injury, and the flames spread to four adjacent buildings.
No other injuries were reported, though there were unconfirmed reports that there may have been squatters inside, as firefighters checked the stability of nearby buildings, including a storefront church next door.
On Monday night, firefighters continued to search for as many as two people missing. They say it may take days to complete the search.
"I'm hoping he's all right. Try to call his phone he ain't picking up try to call his girl's phone he ain't picking up, his girl trying to call him he ain't pickup up, his brother trying to call him he ain't picking up. It's not looking good," said Tyson Williams, a friend of the missing victims.
Earlier, firefighters spent the day dousing hot spots as they investigated the cause.
FDNY Deputy Chief James McNally said the building was fully engulfed within minutes of firefighters' arrival, and he vowed the department will not leave the site "until we get to the foundation and make sure it is searched properly."
Google Street View showed what the building used to look like, littered with trash and graffiti.
Investigators are working to determine if the fire started accidentally or was intentionally set, as well as why the building was allowed to sit unattended for so long.
The Department of Buildings told Eyewitness News it issued multiple stop work orders for the site in recent years because of violations including an unlocked fence.
Neighbors said they have been complaining to the city about the situation for years, and no lasting action was ever taken.
"This building should have been torn down three years ago," said a man named Assad, who lost his business in the fire. "And our insurance lapsed, we don't have insurance."
Neighbors say the neglected building was a disaster waiting to happen.
"Abandoned, they'd work on it, leave it, and come back and work on it and then leave it," neighbor Helen B. said.
But there hadn't been any actual work on the building since last June, when a stop work order was issued.
"I guess people just started getting used to the fact that nobody's telling them anything, and they're just going in there," neighbor Roylan Reina said.
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As the investigation continues, the neighborhood's new City Council member Chi Ossé says it's time to do something abandoned buildings with steeper vacancy taxes.
"So that a lot of these vacant lots can turn into something better for our community," he said. "Whether it's a small business or whether it's affordable housing."
He is also calling for harsher penalties for owners who abandon their properties and disaster happens.
The stop work order on the construction was issued last June after the contractor withdrew from the project.
There were still four open violations having to do with not securing the building.
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