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Crews used generator-powered floodlights and thermal imaging cameras to identify heat spots - bodies or pockets of fire - at what was once 1644 and 1646 Park Avenue, near 116th Street.
Police guarding the scene wore surgical masks and neighborhood residents covered faces with scarfs amid the thick, acrid air.
The explosion injured more than 60 people, with searchers still trying to locate others who remained unaccounted for a day later. Mayor Bill de Blasio said that the city was "hoping to find others still alive" and then "to determine exactly what happened here."
"I can only imagine knowing that at any moment you might find a body, how difficult that is," he told firefighters at the scene.
The explosion on Wednesday morning shattered windows a block away, rained debris onto elevated commuter railroad tracks close by, cast a plume of smoke over the skyline and sent people running into the streets.
Edward Kilduff, the Fire Department's chief of department, said the amount of debris had been reduced to about 1½ floors by Thursday morning. He said rescuers were concerned about the stability of a free-standing wall at the back of the scene.
Scores of residents who lived in or near the two collapsed structures were temporarily relocated to nearby PS 57, where a Red Cross staging area was set up. The displaced have since been relocated to the Salvation Army, where food, shelter and counseling are available, regardless of immigration status.
No name or identifying details were given for the body taken from the rubble Thursday. Of the seven other victims recovered, three women and one man have been identified A handful of people are still unaccounted for and it's still being treated as a rescue mission.
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Meanwhile, the search for clues as to what caused the blast continues.
The preliminary theory is that the buildings were ripped apart by natural gas that built up inside and then exploded. A more than century-old cast-iron pipeline supplied the building, and several families in the buildings smelled gas the night before the explosion.
The fiery blast erupted around 15 minutes after a neighboring resident reported smelling gas, authorities said. The Con Edison utility said it immediately sent workers to check out the report, but they didn't arrive until it was too late.
Fire Commissioner Salvatore Cassano and Con Edison CEO John McAvoy said that before that call, they had received no complaints in the last 30 days about a gas leak in the area.
At least three of the injured were children; one, a 15-year-old boy, was reported in critical condition Thursday with burns, broken bones and internal injuries. A woman was in critical condition with a head injury. Most of the other victims' injuries were minor and included cuts and scrapes.
A tenant in one of the destroyed buildings, Ruben Borrero, said residents had complained to the landlord about smelling gas as recently as Tuesday.
A few weeks ago, Borrero said, city fire officials were called about the odor, which he said was so bad that a tenant on the top floor broke open the door to the roof for ventilation.
"It was unbearable," said Borrero, who lived in a second-floor apartment. "You walk in the front door and you want to turn around and walk directly out."
The Fire Department said a check of its records found no instances in the past month in which tenants of the two buildings reported gas odors or leaks.
Edward Foppiano, a Con Ed senior vice president, said there was only one gas odor complaint on record with the utility from either address, and it was last May, at the building next door to Borrero's. It was a small leak in customer piping and was fixed, he said.
The block was last checked on Feb. 28 as part of a regular leak survey, and no problems were detected, Foppiano said.
Con Ed said it remains to be seen whether the leak was in a company main or in customer-installed inside plumbing. The gas main that serves the area was made of plastic and cast iron, and the iron dated to 1887, Foppiano said.
"Age is not in and of itself an issue with cast iron," he said, noting that Con Edison has a cast iron replacement program and the pipe was not slated to be removed in the next three-year period.
A National Transportation Safety Board team arrived in the evening to investigate. The agency investigates pipeline accidents in addition to transportation disasters.
NTSB team member Robert Sumwalt said investigators would be looking at how Con Edison handles reports of gas odors and issues with the pipe and would be constructing a timeline of events.
Just before the explosion, a resident from a building next to the two that were destroyed reported smelling gas inside his apartment and thought the odor might be coming from outside, Con Ed spokesman Bob McGee said.
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Associated Press contributed to this report.
