Truckloads of scattered material will be sifted for any traces of human remains that might not have been found at the site, said city Fire Commissioner Salvatore Cassano. Although all eight bodies reported missing have been recovered, the rescue operation was continuing in case unknown people were buried beneath the rubble, he said.
Arson detectives and fire marshals were waiting to enter the basements to examine meters, check pipes and inspect any possible ignition sources, such as light switches, that might have caused the blast. About 70 percent of the debris was cleared by midday Friday after a hazardous rear wall that had slowed the process was finally removed.
More than 60 people were injured in the East Harlem explosion, and more than 100 others have been displaced by it, officials said.
The theory that the explosion was due to a gas leak gained momentum Friday after the National Transportation Safety Board, which investigates pipeline accidents, said underground tests conducted in the hours after Wednesday morning's explosion registered high concentrations of natural gas.
NTSB team member Robert Sumwalt said utility Consolidated Edison dug 50 holes about 18 to 24 inches deep around the blast site and measured gas levels in those cavities soon after the explosion. Gas concentration was up to 20 percent in at least five spots, and normal levels in the city's soil should be zero, he said.
"Somehow or another, natural gas did work its way into the ground," he said, adding that workers were testing nearby pipes to identify any with potential leaks.
The NTSB will conduct its own inquiry after police and fire officials determine what might have sparked the blast.
Police have identified six of those who died: Griselde Camacho, 45, a Hunter College security officer; Carmen Tanco, 67, a dental hygienist who participated in church-sponsored medical missions to Africa and the Caribbean; Andreas Panagopoulos, 43, a musician; Rosaura Hernandez, 22, a restaurant cook from Mexico; George Ameado, 44, a handyman who lived in one of the buildings that collapsed; and Alexis Salas, 22, a restaurant worker.
Mexican officials said another Mexican woman, Rosaura Barrios Vazquez, 43, was among those killed.
The name of the eighth person recovered, a woman, hasn't been released.
After touring a Red Cross shelter where some of the displaced residents have been placed temporarily, Mayor Bill de Blasio pledged his support to find suitable temporary or long-term housing options for about 50 families, totaling more than 100 people.
"It's our obligation as the city of New York, and I know all New Yorkers feel this way, to stand by them," he said.
The Department of Homeless Services has about 50 apartments available for families in private buildings where nonprofits are involved in the management, the mayor said. He said officials are arranging for more apartments that would be available for up to three months.
Investigators were trying to pinpoint the gas leak and determine whether it had anything to do with the city's aging gas and water mains, some of which were installed in the 1800s. More than 30,000 miles of decades-old, decaying cast-iron pipe still are being used to deliver gas nationwide, according to U.S. Transportation Department estimates.
Fire and utility officials said that if the buildings were plagued in recent days or weeks by strong gas odors, as some tenants contend, they have no evidence anyone reported it before Wednesday. 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 blast erupted about 15 minutes after someone from a neighboring building reported smelling gas, authorities said. Con Edison said it immediately sent workers to check out the report, but they got there too late.
The lesson, de Blasio said, is that people should heed the post-Sept. 11, 2001, slogan, "If you see something, say something."