A construction crew blamed for digging into a gas line before a home exploded near Hayward this month had dug into a nearby gas line months earlier, according to residents and authorities with knowledge of the case.

Two hours before a home exploded on East Lewelling Boulevard near Hayward, the NBC Bay Area Investigative Unit has learned, a construction crew was using a grinder – a piece of heavy equipment used to remove worn pavement – to remove a 14-foot-wide section of asphalt.

The National Transportation Safety Board says it is preparing a preliminary report but that a ¾-inch diameter steel line dating to the 1940’s was “damaged during grading work associated with a road improvement project along the street in front of the home.”

“When damage like that happens, there’s always a learning experience and one should learn from what happened,” said Ravi Chhatre, a retired NTSB investigator who probed the deadly San Bruno gas explosion in 2010 as well as dozens of other pipeline incidents.

He says contractors should take special care using such heavy equipment to avoid disturbing buried lines. “Typically, near a gas line,” he says, “hand digging is recommended.”

Residents tell the Investigative Unit that Dec. 11 wasn’t the first time workers on the Lewelling project had hit a PG&E line, triggering a leak. Months earlier, there was a leak next door. There’s a patch of new concrete that still marks the spot where PG&E crews dug in front of the business to access the line and cut the flow of gas.

Residents say that PG&E came out after the contractor hit the line while removing a section of concrete sidewalk that was being replaced.

Chhatre called the earlier leak “a big red flag” because it suggests problems with the contractor practices or PG&E records, or both.

The contractor referred questions to the NTSB as did PG&E. An NTSB spokesman said safety investigators are preparing a preliminary report on the explosion that is expected to be released soon

Chhatre said pipeline damage is “unfortunately, more common than anybody would like.”

He recommends utilities and first responders evacuate people near leaks, especially when gas escapes for two hours near homes and businesses, like on Dec. 11.

PG&E and Alameda County fire officials have confirmed no evacuation was initiated that day. Firefighters say the utility told them that it was a small leak and the utility had the situation in hand. PG&E says it is still investigation why no evacuation was called for.

Chhatre says not telling people to leave was clearly a critical oversight – given the speed pressurized gas can travel underground.

“Two hours is a significant amount of time for enough gas to leak and migrate,” he says, adding that the migration continues even after the gas is shut off. So it’s better to play it safe and evacuate.

“Is it an inconvenience?” he asked.

“Yes, but I’m sure a lot of those people who are injured probably would have preferred to get evacuated and find out nothing is wrong, and then be back home…’’