Investigators found evidence of a tampered gas line and adjacent pipe in an East Village building about an hour before the structure blew up -- indications of improper accessing of gas service that led to a deadly, catastrophic fire Thursday, according to a law enforcement source familiar with the case.

The evidence, which the source said involved a larger main line, was noticed by Con Edison inspectors during the final visit they made to 121 Second Ave. on Thursday, when the utility said it found that improper meter placement in the basement prevented gas service from being given to upstairs apartments. A ground-floor Japanese restaurant was authorized to have service through a smaller line, officials said.

A Con Edison spokesman was unable to respond late Tuesday to what was discovered during the final inspection. The source said the problems found were a pipe with a clamp and a lock on the unused large main that may have been tampered with. In earlier statements, Con Edison said the gas service had been "locked" to prevent use.

StorySource: Tenants in blast didn't have OK to use gas

Investigators with the NYPD, FDNY and Manhattan district attorney's office are probing the cause of the gas explosion that destroyed three buildings, killed two people, injured 22 and disrupted the neighborhood.

A number of law enforcement sources said Tuesday the investigation could take several weeks and possibly a year to learn what caused the explosion and whether criminal charges are warranted. Investigators may need to find a cooperating witness in the case, one of the sources said.

A law enforcement source said earlier that investigators suspect that someone may have told residents in the building that gas service would be shut off temporarily the day of the explosion and to not tell Con Edison they had gas. Calls to building owner MAH Realty LLC weren't returned Tuesday.

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The strength of the blast and the intense fire indicate that the larger 3-inch main was leaking, said Mark McDonald, a forensic gas expert not involved in the case.

"It wasn't a leak on a fitting," McDonald said. "This was a lot of gas, a pipe, a break in a line, [an] open pipe without [a] plug."