The water main break helped create a large sinkhole in front of the buildings, and searchers were unable to use heavy machinery until it was fixed.
Officials need to determine if the water main broke before or after the explosion.
Approximately 17 minutes before the explosion, a nearby resident had called Con Ed to report smelling gas.
Within hours, investigators were focusing in on an old cast-iron gas pipeline as a possible cause.
"We have a long standing concern about cast iron pipes and we are certainly looking at that," said Robert Sumwalt, NTSB.
Pipeline safety data show that Con Ed hasn't had a fatal gas line incident in five years back when this 2009 Queens explosion killed a mother of three.
In 2008, an explosion at a Queens apartment building killed one and injured 17.
And the year before, there was another fatality when a ruptured Con Ed gas line leveled a home in Sunnyside, Queens.
Before today's blast, some residents claimed to have smelled a strong odor of gas.
"As soon as you walk in front of the hallway, you could barely get to your door it was so bad, the smell of gas," said Ruben Borrero, a resident.
Con Ed says they only ever received one call about the smell of gas and that was Wednesday morning about 20 minutes before the blast.
The NTSB plans to look closely at the call records.
"We will be looking at all reports, we'll be looking at Con Ed's call logs to see when the first calls started coming in to report this, and that definitely will be part of the investigation," Sumwalt said.
So too will the cast-iron gas pipeline. A picture shows how brittle these 100-plus year lines can be, especially in harsh winters. It's why Con Ed and other utilities have been under pressure to replace them.
"Right now is a very tricky time, very dangerous time, for gas distribution systems with lots of cast-iron," said Bob Ackley, of Gas Safety USA.