New York Mayor Bill de Blasio told a news conference that preliminary evidence indicated it was a gas-related explosion.
A Fire Department spokesman said late on Thursday that three of the four buildings had collapsed or partially collapsed.
The fourth building "is still involved with some pockets of fire, it`s still an active scene. Investigation is still ongoing," he said.
Nineteen people were hurt. Four, all civilians, were in critical condition, the spokesman said.
The Con Edison (ED.N) utility said in a statement that its inspectors had been at the site on Thursday to evaluate work a building plumber was doing in a building for a gas service upgrade. The work failed to pass inspection, it said.
The seven-alarm blaze in the neighborhood of small businesses, restaurants and apartments involved more than 250 Fire Department personnel.
Moishe Perl, 64, who owns Moishe’s Bake Shop nearby, said he heard an explosion, ran outside and saw the lower floors of a building start to crumble.
"Most of the people were running out of the building and climbing down the fire escape," he said, while others were helped out of windows by passersby.
Ben Mackinnon, 28, said he was drinking coffee in a cafe when he heard an explosion from across the street.
"The explosion was big enough that the door of the cafe blew open," Mackinnon said.
He said he saw several bloodied men emerge from a sushi restaurant where the explosion appeared to originate. One of them fell to the pavement.
Shameem Noor, a cashier at the Veselka restaurant about a block away, said he heard the blast and saw three or four people fall to the street.
The four buildings contain 49 apartments, according to a spokesman for the American Red Cross at the scene. The ground floors were occupied by small eateries.
A relocation center for displaced residents was set up in a nearby elementary school, the Red Cross spokesman said.
Early on Friday morning, firefighters were battling a separate apartment blaze burning on the top floor of a six-story building some 3 miles (5 km) north, the New York Fire Department said. There were no immediate reports of injuries.
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