Someone possibly "inappropriately accessed" a gas line, causing an explosion and fire that destroyed a row of East Village tenements, injuring 22 people, four of them critically, and leaving at least two people unaccounted for, Mayor Bill de Blasio said Friday.

The mayor said the owner of a sushi restaurant on the ground floor of the building at 121 Second Ave. smelled gas just 15 minutes before the explosion, but called the landlord instead of authorities.

De Blasio said investigators are working on the theory that someone "inappropriately accessed" the gas line in the basement, but would not explain what prompted that suspicion.

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"The strong assumption is a gas explosion; we are waiting for details," de Blasio told reporters at a briefing Friday in lower Manhattan. "There is a possibility here that the gas line was inappropriately accessed internally by people in the building."

He added: "There is some X factor we don't know yet we have to find."

Authorities identified two unaccounted for men who are believed to have been in the sushi restaurant at the time of the blast as Nicholas Figueroa, 23, whose family said he was on a date in the eatery, and Moises Lucon, a restaurant worker. Figueroa's family declined to comment as they left a nearby assistance center for victims of the blast.

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Con Ed president Craig Ivey said the building's current gas line was only for the restaurant. Employees of a private contractor were adding a new plumbing and gas system to an existing one inside the sushi restaurant, authorities said.

They had permits to do the work and Con Ed inspectors visited the site an hour before the explosion and found "insufficient spacing" for the installation of a meter in the basement, and other issues, a Con Ed spokesman said. Inspectors left at 2:45 p.m. and the owner reported the smell of gas about 3 p.m., officials said.

"In fact, the reason we put in the larger line, the smaller line didn't have the capacity to serve the entirety of the building," Ivey said. Asked how the building's four occupied apartments were accessing gas, he said: "That's a great question."

The explosion caused the collapse of three five-story buildings and damage to a fourth, which was seven stories tall, the FDNY said.

The mayor said six firefighters were among the injured, but none of the injuries was life-threatening. A highly decorated off-duty firefighter was credited with climbing a fire escape to assist victims, officials said.

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The mayor and other officials walked through the mounds of debris Friday morning as half-a-dozen pockets of smoke or steam rose from the rubble.

Joseph Esposito, head of the city's Office of Emergency Management, told reporters after the mayor left that it was too early to say exactly what had happened, and workers could not begin examining the interior until the fire was out.

Asked if there was any chance of survivors being found on the site, Esposito said: "I would doubt that very seriously."

The city provided temporary housing for 30 of the 84 people who registered for help.

A Con Ed statement Thursday said the utility "had no reports of gas odors in the area before the fire and explosion. A survey conducted yesterday [Wednesday] of the gas mains on the block found no leaks."

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The general contractor working at the building at the time of the explosion, according to city officials, was Dilber Kukic, who was charged last month by the Manhattan district attorney's office with trying to bribe an undercover officer posing as a corrupt Department of Buildings inspector. Kukic, officials said, paid an undercover $500 to remove violations on a building he owned on West 173rd Street and $2,500 to get rid of violations on other properties he owns.

But Chief of Detectives Robert Boyce said Kukic, who was injured in the blast, has been very helpful in the building explosion investigation and helped pull the owner's son from the blaze.

The explosion was heard about 3:15 p.m. Thursday. A Con Ed inspection at 2 p.m. gave failing marks to the new work, preventing any gas from flowing through the new lines, authorities said.

The first of 250 firefighters arrived three minutes after the first 911 call came at 3:17 p.m., said FDNY Commissioner Daniel Nigro: "They certainly didn't expect to see the explosion blow the front of 121 across the street." Of the injured firefighters, two had burns to their airways and one was knocked unconscious, Nigro said.

Con Edison crews shut off gas to the area, and Gov. Andrew M. Cuomo said the state Department of Public Service was on the scene to monitor the utility's performance.

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He also said the state agency will conduct a "full investigation" to determine the cause of the explosion.

"We will continue to monitor this tragedy and do whatever is needed to support the ongoing response and recovery in the days ahead," Cuomo said in a statement.

With William Murphy

and Anthony M. DeStefano