City officials say 60 to 70 percent of the rubble has been removed from Wednesday's explosion and building collapse in East Harlem and say all missing persons have likely been accounted for.
The death toll from the collapse stands at eight.
The rubble will be taken to Randall's Island for forensic examination.
The building's back wall was taken down because it was unstable Friday, creating dangerous conditions for workers.
Crews say they hope to make it down to the lower levels on Saturday.
Once they get to the basements, fire officials will begin to try and piece together the cause.
Workers have also removed a number of crushed vehicles from the area, and they say that several people were in them at the time and had to be pulled to safety, including an elderly woman in an ambulette.
Fire Commissioner Salvatore Cassano said that they are going through each piece of the rubble to try and piece together the cause of the explosion.
"We'll take the debris from the area, put it on the sidewalk, examine it before we take it to Randall's Island, where the debris will be saved to do a forensic study as part of the investigation," Cassano said.
Federal investigators say that there was natural gas in the ground in the area of the building collapse that should not have been there.
The National Transportation Safety Board said that they found out from Con Edison that after the explosion, the utility came in and tested the ground for natural gas, and that at at least five of those locations, they found that there was anywhere between 5 and 20 percent of natural gas in the ground.
"Normally, the soil in New York City 18 to 24 inches down into the ground would have zero concentration of natural gas, so the fact that in least five of the holes, the concentration on gas ranged between 5 and 20 percent, that tells us that's a pretty good concentration of natural gas in that area," said Robert Sumwalt of the NTSB. "That further leads to the hypothesis that this may well have been a natural gas leak."
The NTSB said that that is a problem, and the investigation will now try to figure out where it is coming from.
The agency said that it will be looking at gas pipelines along Park Avenue, as well as 116th Street, doing those pressure tests, where they will cap off the area and try to find out how much gas is leaking and from what area of the pipe.
Mayor Bill de Blasio said that the city is working to find temporary housing for those displaced and will not leave anyone behind.
De Blasio met with some of those displaced families Friday.
The mayor's office said that the Real Estate Board has 15 apartments set aside on the East Side for the next three months.
The board said it's also working with other agencies on temporary and long-term housing.
"We made clear to them that we will stand by them every step of the way. We will not let them fall. They've been through the unspeakable. And obviously, we met people who have lost family members and neighbors all in an instant, with no warning. It's our obligation as the City of the New York - I know all New Yorkers feel this way - to stand by them," de Blasio said.
"There's been permanent displacement," said City Council Speaker Melissa Mark-Viverito. "The two buildings that collapsed, we had about 15 units of housing, so obviously, those families need to be provided assistance and support, and we need to provide permanent housing. And then also, you've got the building right adjacent to the explosion site. Those families have been temporarily displaced."
The mayor's office said that 91 apartments in surrounding buildings have been evacuated while cleanup and inspections continue.
Meantime, all but one of the victims has been identified.
The family of 44-year-old George Amadeo tells Univision they believe he died while trying to rescue his dog Blackie.
They say he lived at 1646 Park Avenue with his mother, who was not home at the time.
The other victims have been identified as 44-year old Andreas Panagopoulos, a Greek musician, 44-year-old Griselde Camacho, a public safety officer at Hunter College, 67-year-old Carmen Tanco, a dental hygienist, Rosaura Hernandez-Barrios, her mother, Rosaura Barrios and 22-year-old Alexis "Jordy" Salas, a junior at John Jay College of Criminal Justice.
Cassano said that plans are already underway to review gas leak response procedures with Con Ed and the city's other natural gas distributor, National Grid.
He said his staff will work with both gas companies in the coming days to assess protocol for response to gas leaks.
Routine reports of gas leaks are handled by the utility crews, but when a pipe is knowingly damaged, or if there are reports of gas from several residents at once, the response can include an immediate dispatch of firefighters.
Cassano said discussions are already underway to determine how the existing protocol for handling gas leak reports can be improved.
Meanwhile, former President Bill Clinton toured the site of the explosion Friday.
The former president surveyed the wreckage and commended rescue and construction workers on their continuing efforts in restoring area.
Clinton keeps an office just a few blocks from where the blast happened.