NEW YORK – Search crews picked through still-smoldering rubble on Thursday, looking for survivors of a gas explosion that caused the collapse of two New York apartment buildings a day earlier, killing seven people and injuring about 60.

Facing thick smoke and bitter cold, dozens of firefighters, police officers and a team from the National Transportation Safety Board were at the Upper Manhattan scene to determine what had caused the explosion on Wednesday, shortly after a resident nearby had called the utility, Con Edison, to complain about the smell of gas. The safety board investigates accidents involving natural gas.

Five people remained missing on Thursday morning, New York Police Detective Martin Speechley said.

Mayor Bill de Blasio met with first responders as heavy-duty evacuation equipment, including a machine known as “the grappler,” moved rubble that was being hosed down by fire trucks.

“I can only imagine knowing that at any moment you might find a body, how difficult that is,” de Blasio told rescue crews. “I admire the work you guys do… Thank God you do it.”

The debris continued to flare up at times as rescuers clawed through the wreckage of the adjoining buildings that had housed 15 apartments on a largely residential block at East 116th Street and Park Avenue.

Passersby wore dust masks or wrapped their faces in winter scarves to limit inhalation of dust and smoke.

The mayor said a preliminary investigation indicated the explosion in East Harlem was caused by a gas leak.

Gas in the area had been turned off.

Amid a return to frigid temperatures, about 69 people who had to vacate their nearby homes on Wednesday due to smoke took shelter overnight at a school, said the Red Cross and the Office of Emergency Management.

Authorities allowed some residents to return to their nearby homes to retrieve possessions, but many left quickly, complaining of the smoke.

Four women and three men were killed in the collapse of the buildings. One victim was identified as Griselde Camacho, a public safety officer for Hunter College in East Harlem, according to a message on the school’s website.

Most of the 60 people wounded suffered from cuts, broken bones or smoke inhalation, police said.

At least three children were among those hurt. Two were treated for minor injuries and released, while a third was in critical condition, hospital officials said at a news conference.

The blast leveled the apartment buildings, located above a ground-floor church and a piano store, sending a cascade of twisted and burnt metal on to the sidewalk and burying parked cars.

The last time the utility had received a complaint about a gas odor in the neighborhood was in May, said Consolidated Edison Inc spokesman Bob McGee.

At the time, Con Ed shut the gas off and the building had hired its own contractor to fix a leak. On July 3, Con Ed crews returned to the building to certify the repairs were done correctly, McGee said.

On February 10 and February 28, there were “high speed” checks made of the gas pipes, he said.