The three-storey building gave way while construction workers were digging at an adjacent lot, causing New York City's mayor to order inspections at about 100 building sites involving companies related to the project.
"If there is anyone, or any entity, operating in this city that bears responsibility for what happened, we will hold them fully accountable," Mayor Zohran Mamdani said during a news conference.
Investigators have not determined what caused the collapse. Two children, ages 9 and 10, were killed along with adults ranging in age from 22 to 31, according to the New York City Police Department.
There were also 18 injured, including four firefighters, Mamdani said. The children were Colombian, as was one of the adults who was killed, according to the country's consulate in New York.
Firefighters pulled a pregnant woman from the rubble and rushed her to a waiting ambulance. Others were rescued as the night wore on.
One witness said he watched bricks fall minutes before the disaster while a woman in a neighbouring building said she and fellow residents blamed vibrations from the construction for cracks that appeared in their apartment walls.
Emergency workers used power saws and pickaxes to get through the debris, pulling four victims out of the rubble in the early hours of Thursday. Hours later, excavators removed larger chunks of the wreckage so searchers could get deeper into the construction pit, said New York Fire Commissioner Lillian Bonsignore.
James Williams, who works across the street, said he heard loud banging at the site all day and looked outside to see bricks falling. "Two minutes later, the building collapsed," he said.
As the building crumbled and exposed the neighbouring building, Williams spotted a woman on a staircase. "There wasn't anything I could do," he said, his voice cracking.
Dervie Mapp, who lives along the construction site, said she and others complained for months about the project. Her landlord brought engineers into the building, but residents never received an inspection report, she said.
Construction safety expert Glenn Corbett of John Jay College in New York said it's way too early to know what caused the accident.
"It could be a design flaw, it could be a construction flaw, or it could be some kind of work that was going on," he said.
It appeared that the shoring of the excavation work failed, but finding out why will take more investigating, he said.
Building records show there were four apartments on the upper two floors and a commercial unit on the ground floor of the collapsed building.
Some of the victims were from Mexico, Colombia, Ecuador and Trinidad, New York's mayor said. Two of the dead were construction workers, he said.
New York City's buildings commissioner, Ahmed Tigani, said Wednesday that construction workers were excavating the empty lot next door when the building buckled. The city's Department of Buildings is investigating.
City officials said a stop work order was issued in March after inspectors concluded the building next door was being demolished without adequate overhead protection such as netting to shield neighbouring buildings, the department said in an emailed statement.
In April, inspectors found that the demolition work had resumed in violation of the stop work order, the department said. Work was allowed to continue after the problems were corrected, it said.
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