Arson detectives and fire marshals have been 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.
The theory that the explosion was due to a gas leak gained momentum Friday after the NTSB, which investigates pipeline accidents, said underground tests conducted in the hours after the explosion registered high concentrations of natural gas. The NTSB will conduct its own inquiry after police and fire officials determine what might have caused the blast.
Cassano said about 15 percent of the debris was left on the site late Saturday afternoon.
An uplifting moment from the painstaking recovery effort came as crews pulled a large water-damaged Bible from the rubble of the Spanish Christian Church, which was located in one of the two destroyed buildings. About two dozen people, including clergy members, carried the Bible in a solemn procession near the East Harlem site.
"This was in the depths of the rubble. Somehow God protected it," said Rick del Rio, a bishop at the Church of God.
The church's pastor, Thomas Perez, suffered heart palpitations when he saw the Bible, said Letitia James, the city's public advocate. He was taken by ambulance to a hospital as a precaution, supporters said.
Cassano said Perez was overwhelmed with emotion.
"It was singed but it meant an awful lot to the pastor," he said. "It showed the pastor that they'll be rebuilding."
Truckloads of scattered material will be sifted for any traces of human remains that might not have been found at the site, Cassano said. Although the bodies of all eight people reported missing after Wednesday's blast have been recovered, the rescue operation was continuing in case others may be buried beneath the rubble, he said.
More than 60 people were injured in the explosion, and more than 100 others were displaced.
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 those displaced.
"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.