The body of unidentified eighth person was pulled from the rubble on Thursday.
At least three of the injured were children. One, a 15-year-old boy, was reported in critical condition with burns, broken bones and internal injuries.
The blast erupted about 15 minutes after someone from a neighboring building reported smelling gas, authorities said. The Con Edison utility said it immediately sent workers to check out the report, but they got there too late.
Con Ed CEO John McAvoy said the call had been correctly categorized as 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 neighborhood residents said they smelled gas on Tuesday but didn't report it. A tenant in one of the destroyed buildings, Ruben Borrero, said that residents had complained to the landlord about the gas odors on Tuesday and that fire officials were also called a few weeks ago.
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.
An Associated Press analysis of the city's 311 calls database from Jan. 1, 2013, through Tuesday also found no calls from the buildings about gas.
The lesson, De Blasio said, is that because of the city's old and vulnerable infrastructure, people should heed the post-Sept. 11, 2001, slogan, "If you see something, say something."
Sumwalt said the NTSB would be checking calls to the city's 911 emergency line and 311 information line and interviewing witnesses, first responders, the injured and those who smelled gas.
The working-class neighborhood around the site at Park Avenue and 116th Street was once known as Spanish Harlem because of its large population of Puerto Ricans but now has many Asians and other ethnic groups. The neighborhood is gentrifying but still has a high crime rate, fueled by drugs and gangs.
Storefronts range from fast-food shops to botanicas selling folk medicine and religious items.
More than 30,000 miles 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 $82 billion.
New York City still uses about 3,000 miles of old cast iron, Boston about 2,000 miles, Philadelphia about 1,500 and Washington 400, the department said. Experts said much of the pipe dates to before World War II, and some of it may even be more than 100 years old.
Associated Press writers Julie Walker, Jonathan Lemire, Jake Pearson, David B. Caruso, David Crary, Leanne Italie, Karen Matthews, Deepti Hajela, Jim Fitzgerald and Mike Casey contributed to this report.