Rescue workers using dogs and thermal-detection gear are searching the rubble for victims of a gas explosion that killed at least seven people, while New York City investigators tried to find the leak and determine whether it had anything to do with ageing gas and water mains, some dating to the 19th century.
At least five people were unaccounted for after the deafening blast on Wednesday morning (local time) destroyed two five-storey apartment buildings in the historically Hispanic neighbourhood of East Harlem. More than 60 people were injured.
Fire and utility officials said that if the buildings in recent days or weeks had strong gas odours, as some tenants claimed, they have no evidence anyone reported it before Wednesday, just minutes before the blast.
Fire Commissioner Salvatore Cassano said experts would first try to reach the basement - still under rubble - to examine heating units, meters and other equipment that might hold clues to the blast. Then they will work their way toward the street, where utility Con Edison has a gas main consisting partly of cast iron from 1887.
"We can only get conclusive evidence when the fire is out, when the rescue is completed, and we really get a chance to look at all the facts," Mayor Bill de Blasio said.
Ageing infrastructure - crumbling bridges, highways, water mains and gas lines - has become a major concern in recent years, especially in older cities in the US Northeast, and has been blamed for explosions, floods and other accidents. But many cities say they just don't have the money to fix the problem.
"We know this is a fundamental challenge for New York City and any older city," de Blasio said.
As below-freezing winds blew across the still-smouldering debris, construction equipment with iron jaws picked up the rubble, first depositing it on the pavement, then hoisting it onto trucks that hauled it away. Clouds of thick smoke swirled over Park Avenue near a busy commuter train line.
Police identified four of the dead: Griselde Camacho, 45; Carmen Tanco, 67; Andreas Panagopoulos; and Rosaura Hernandez, 22, of Mexico.
Mexican officials said another Mexican woman, Rosaura Barrios Vazquez, 43, was among the dead.
The bodies of three unidentified men also were pulled from the rubble.
At least three of the injured were children. One, a 15-year-old boy, was reported in critical condition on Thursday with burns, broken bones and internal injuries. A woman was in critical condition with a head injury.
The blast erupted about 15 minutes after someone from a neighbouring building reported smelling gas, authorities said. Con Ed said it immediately sent workers to check out the report, but they got there too late.
Con Ed chief executive John McAvoy said the call had been correctly categorised as a low priority. "A single person calling that they smelled gas outside of a building is not something that would warrant a Fire Department response," he said.
After the disaster, a number of neighbourhood residents said they smelled gas on Tuesday but did not report it. A tenant in one of the destroyed buildings, Ruben Borrero, said residents had complained to the landlord about gas odours on Tuesday and that fire officials were called a few weeks ago.
"It was unbearable," said Borrero, who lived in a second-floor apartment with his mother and sister, who were away at the time of the explosion. "You walk in the front door and you want to turn around and walk directly out."
But Cassano and McAvoy said that before Wednesday, the Fire Department and Con Ed had received no complaints in the last 30 days about a gas leak in the area.
The working-class neighbourhood was once known as Spanish Harlem because of its large population of Puerto Ricans but now has many Asians and other ethnic groups.
The explosion destroyed everything Borrero's family owned, including the ashes of his father, who died a few years ago.
But "I have my mother and sister," he said. "I'm happy for that."
More than 48,280 kilometres of decades-old, decaying cast-iron pipe are still being used to deliver gas nationwide, according to the U.S. Transportation Department estimates. In 2011, the American Gas Association said replacement or repair could cost US$82 billion (NZ$95.72bn).
New York City still uses about 4828km of decades-old cast-iron pipe.