Emergency workers are searching for at least two people still missing after an apparent gas blast destroyed three New York apartment buildings.
Investigators are working to establish what exactly caused the explosion that injured 22.
Sniffer dogs have been brought in to hunt for anyone trapped beneath the heap of loose brick and rubble.
Detectives have issued posters seeking information on the whereabouts of two men believed to have been in a sushi restaurant on the ground-floor of one of the now-collapsed buildings: a 26-year-old restaurant employee and 23-year-old bowling alley worker who had been there on a date.
Authorities also were exploring whether a third person was unaccounted for, chief of detectives Robert Boyce said.
"There's a lot more we need to learn," New York mayor Bill de Blasio said, a day after the blast in Manhattan's East Village neighbourhood.
It's possible that someone improperly tapped a gas line amid ongoing plumbing and gas work in one of the destroyed buildings, though investigators need to get into the basement to learn more, Mr de Blasio said.
Consolidated Edison utility inspectors visited the work site on Thursday, about an hour before the explosion and determined work to upgrade gas service did not pass muster, locking the line to ensure it would not be used before leaving, officials said.
But 15 minutes later, the sushi restaurant owner smelled gas and called the landlord, who called the general contractor, Mr Boyce said. Nobody called emergency services or Con Ed.
The contractor, Dilber Kukic, and the owner's son went into the basement and opened a door, then the explosion happened, burning their faces. Mr Kukic, 39, who is facing an unrelated bribery charge, declined through his lawyer to comment on the circumstances surrounding the explosion.
The building had an existing gas line intended to serve the sushi restaurant; work was under way was to put in a bigger line to serve the entire building, Con Ed president Craig Ivey said. Whether the largely vacant apartments were getting gas from the existing line was "a great question", he said.
Mr de Blasio would not say more about why officials believe the existing gas line might have been tapped. But the building had a history: Con Ed found an unauthorised gas pipe there in August after getting a report of a gas smell, according to a city official briefed on the information.
The pipe was gone when Con Ed checked again 10 days later and some of the people involved with the building were not co-operating with investigators, the official said.
Dozens of neighbours have been evacuated because of the blast and a fourth building badly burned after the explosion remained at risk of potentially collapsing, authorities said.