The explosion reportedly occurred after the building's owner and a contractor arrived to investigate the smell of gas reported by a restaurant owner in one of the buildings, police said, following a gas company inspection of a contractor's work in a basement nearby.
In all, three buildings collapsed and a fourth building was seriously damaged, New York City Mayor Bill De Blasio said, adding that the fire department is still actively fighting the fire. Though "the strong assumption is a gas explosion" was the cause, no final conclusion has been drawn, he added.
"There is a possibility here that the gas line was inappropriately accessed internally by people in the building," DeBlasio said. "We don't know enough yet, though, about the details of that."
DeBlasio said investigators may not be able to determine any more until they clear the debris and access one of the destroyed buildings.
"There's certainly a possibility of impropriety, but until we access the site we cannot get the next step," he said. "Until we get into that basement and get the full picture, we will not be able to confirm [the explosion's cause]."
Twenty-two people were injured, four critically, De construction videos Blasio said today. No fatalities have been reported, but two people remain unaccounted for.
Two of the most seriously injured victims were Michael Hrynenko, a building co-owner, and Dilber Kukic, a general contractor working at the site who helped carry Hrynenko construction careers away from the blast, police said. Both were being treated at New York Presbyterian Hospital's burn unit.
The two men had earlier figured in events leading up to the blast, according to authorities.
Inspectors from the local power and gas company, Con Ed, arrived at the building at 2 p.m. Thursday to meet with the contractor to evaluate work being done by a plumber in the basement of one of the buildings, according to police. The work failed inspection.
After the inspectors left, the owner of the Sushi Park restaurant, located in one of the buildings, notified a building owner, Maria Hrynenko, mother of Michael Hrynenko, of a gas odor, according to police. Michael Hrynenko and Kukic met up at the location to investigate, and when they opened the side door of 119 Second Avenue, also known as 45 East 7th Street, there was an explosion, officials said.
Officials said Kukic was answering questions and helping with the investigation.
New York City Mayor Bill de Blasio visits the site of a seven-alarm fire that caused the collapse of two buildings and damage to two other buildings a day after the blaze took place, March 27, 2015.
The Department of Buildings will be inspecting all of the surrounding buildings before they are re-inhabited, the mayor said.
The gas is shut off for the immediate surrounding area and air construction videos quality has returned to normal levels in the surrounding area.
"Once the debris is cleared and the fire is fully extinguished then we will do a full investigation and clean-up," De Blasio said, adding that the full investigation will "take days."
He noted repeatedly that people who detect the smell of gas should not investigate it themselves, but call the proper authorities immediately.
"It could have happened in any building," the mayor said. "The important thing is for people to call 911 or Con Ed."
ABC News' Josh Margolin, Michael S. James and WABC-TV contributed to this report.