More than 60 people were injured. Investigators were trying to pinpoint the leak and determine whether it had anything to do with the city's aging gas and water mains, some from the 1800s.
Fire and utility officials said that if the buildings were plagued in recent days or weeks by strong gas odors, as some tenants contended, they have no evidence anyone reported it before Wednesday.
The gas main and distribution pipe under the street had been examined in a crater and were found to be intact, with no obvious punctures or ruptures, Sumwalt said Thursday. They had not been torn from the ground.
However, he said NTSB investigators had been unable to conduct a fuller examination because of the rescue effort underway, and it was unclear whether the leak came from inside or outside the buildings.
Sumwalt said there had also been a water main break at the site, but it was unknown if that contributed to the gas explosion or was caused by it. The water main was installed in 1897, according to the city.
The NTSB investigates pipeline accidents in addition to transportation disasters.
Authorities hoped to reach the basement — still buried under rubble — to examine heating units, meters and other equipment that might hold clues to the blast, Fire Commissioner Salvatore Cassano said.
Mayor Bill de Blasio said: "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."
Aging infrastructure — crumbling bridges, highways, water mains and gas lines — has become a major concern in recent years, especially in older cities in the Northeast, and has been blamed for explosions, floods and other accidents.
"We know this is a fundamental challenge for New York City and any older city," de Blasio said. But he said the federal government needs to provide more aid to cities to deal with the problem.