By Chris Francescani and Victoria Cavaliere
NEW YORK, March 13 (Reuters) - Search crews picked through still-smoldering rubble on Thursday, looking for survivors of a gas explosion that caused the collapse a day earlier of two New York City apartment buildings, killing seven people and injuring about 40.
Facing thick smoke and braving bitter cold, dozens of firefighters, police officers and federal investigators were at the scene in Upper Manhattan to determine what caused the explosion at about 9:30 a.m. (13:30 GMT) on Wednesday.
The blast shook the East Harlem neighborhood shortly after a resident complained to the Con Edison utility about a gas odor. The National Transportation Safety Board, which reviews accidents involving natural gas, was also at the site.
"It's been a painful 26 hours now," Mayor Bill de Blasio told a press conference, lamenting that the neighbor who called Con Ed at 9:13 a.m. (13:13 GMT) on Wednesday failed to call the night before, when he first noticed it.
"That might have given us an opportunity here," de Blasio said, urging anyone who smells gas to immediately call Con Edison or the city's complaint line at 311.
De Blasio, said he expected it would be days before the rescue and cleanup work would be completed, allowing investigators to get into the basement of the buildings to better determine the cause of the blast, earlier believed to have been a gas leak.
Earlier on Thursday, the mayor met with first responders as heavy-duty evacuation equipment, including a machine known as "the grappler," removed rubble that was being hosed down by fire trucks.
A large sink hole, caused by a water main break in front of the buildings, had delayed the equipment from reaching the site on Wednesday.
"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 two structures that had housed 15 apartments on a largely residential block at East 116th Street and Park Avenue.
Passersby in the primarily Latino neighborhood wore dust masks or wrapped winter scarves around their faces to limit inhalation of dust and smoke.
Gas in the area has been turned off. After a return to frigid temperatures, seven buildings with no gas or heat were vacated and 66 people have been given shelter, de Blasio said.
"Anyone affected by this tragedy will be helped - anyone - regardless of immigration status... They should not be afraid," de Blasio said.
Authorities allowed some residents to return to their homes to retrieve possessions, but many left quickly, complaining of the smoke.
Four women and three men were killed, but only two victims' identities have been released - Griselde Camacho, a public safety officer for Hunter College in East Harlem, and Carmen Tanco, a dental hygienist.
De Blasio declined to say how many people remain unaccounted for, although New York City police earlier said five are still missing.
The mayor said 40 people suffered injuries. Police, who earlier on Thursday put that number at 60, saying that most of them had cuts, broken bones or smoke inhalation.
At least three children were 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 buildings, which had apartments above a ground-floor church and a piano store, scattering twisted and charred metal on the sidewalks and burying parked cars.
Consolidated Edison Inc spokesman Bob McGee said the last time the utility had received a complaint about a gas odor in the neighborhood was in May.
At the time, Con Ed shut off the gas and the building hired its own contractor to fix the leak. On July 3, Con Ed crews returned to the building to certify that repairs were done correctly, McGee said.
On Feb. 10 and Feb. 28, there were "high speed" checks made of the gas pipes, he said.