Investigators suspect tenants in a building that exploded Thursday may have been told not to alert Con Edison inspectors before the blast that they had gas service, despite only a restaurant on the ground floor being cleared to have it, said a law enforcement source familiar with the case.

The residents of 121 Second Ave. were also allegedly told their gas service would be temporarily shut off before the Con Ed inspectors arrived Thursday to check contracting work in the restaurant, said the official, who asked not to be identified. Turning off the gas appears to have been an attempt to deceive the inspectors, the source said. It was not clear Monday who investigators believe talked to the tenants.

In a statement after the blast, Con Ed officials said gas service in the building had been "locked to ensure it would not be used." It was unclear when the utility turned off the gas. The building's only permitted gas line serviced the sushi restaurant where contractors were adding a new plumbing and gas system before the explosion, authorities said.

The gas explosion took down the Second Avenue structure and two adjacent buildings, killing two men, injuring 22 and displacing hundreds.

There was evidence that the gas line had been improperly accessed to provide surreptitious service to the building, Mayor Bill de Blasio said Friday. Investigators Monday were still digging through rubble to the basement in order to conduct a full forensic investigation.

One of two bodies found inside the rubble Sunday was identified Monday by the mayor's office as Nicholas Figueroa, 23, who was on a date inside the restaurant at the time of the blast.

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After the explosion, Figueroa and restaurant worker Moises Locon were reported missing. Officials have not identified the other body, but FDNY Commissioner Daniel Nigro said Sunday that it was likely Locon.

The case is considered a "suspicious death" investigation but could escalate into a homicide case once investigators examine the basement, the source said.

Real estate records show that 121 Second Ave. is owned by MAH Realty LLC. Repeated calls to principals of MAH have not been returned during the past several days.

Until investigators are able to examine the basement gas meter and pipes, the explosion's cause or whether there was negligence will remain a mystery, the official said. A spokeswoman for the Manhattan district attorney's office declined to comment on the case Monday.

Con Edison has said in statements that the work done by contractors failed two inspections, including one done less than an hour before the blast. Among the problems found were improper placement of the meter, Con Edison officials said. A spokesman for the utility declined to say what other problems were found on the visit Thursday by inspectors or whether they noticed evidence of illegal tampering.

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After Con Edison inspectors left the building, the manager of the restaurant smelled gas and called the building owner instead of Con Edison. Dilber Kukic, a contractor doing work in the building, entered the basement to investigate, officials said, adding that after he opened the door leading to the gas meter, the building exploded. Kukic survived.

A forensic gas investigator who didn't want to be identified said that gas can accumulate in a room and become too rich to ignite. But additional air -- such as through the opening of a door -- lowers the gas concentration to an explosive level.