The deadly gas explosions that rocked three communities north of Boston last week were linked to an overpressure in pipes owned by Columbia Gas construction, officials said Sunday.
"We found evidence from an evidentiary dig this morning that pressure sensors were attached to a gas line that was being placed out of service and being capped off on Thursday," the National Transportation Safety Board Chairman Robert Sumwalt said during a briefing.
The exact cause of the pressure increase is still unknown, the NTSB said. Authorities cautioned that while they work to pinpoint the exact cause of the blasts in the Merrimack Valley, residents should remain vigilant and report if they smell gas.
Dozens of homes were destroyed or damaged, a teenager was killed and dozens of people were injured in Lawrence, North Andover and Andover. Thousands were forced to leave their homes.
On Sunday, those residents were allowed to return to their homes after crews finished shutting off nearly 8,600 gas meters. Electricity was restored to all the affected homes and businesses, but gas service may not be restored for weeks.
Returning residents had to wait in long lines at the Columbia Gas claims center in Lawrence where they were able to get gift cards for basic goods like groceries -- a welcome help-line for families who lost everything.
Similarly, a recovery center was set up at Arlington Middle School, where donations are coming in from all over.
"It's evident to me and to all of us the Merrimack Valley and the residents of our state are being as supportive as they can be and as kind as they can be to one another during this most difficult time," Gov. Charlie Baker said at a news conference Sunday. "We still have a very long way to go but we're very happy that people can return to their homes this morning."
Schools in Lawrence will be closed Monday because of the natural gas situation, officials said.
Sumwalt said the investigation is focused on high pressure in pipelines. He said officials will be looking at how local Columbia Gas officials responded to a "pressure increase" in the Lawrence area that was detected at the company's pipeline control center in Columbus, Ohio, prior to the explosions and fires. He said there was no evidence the explosions were intentional.
Columbia Gas on Sunday turned away hundreds of Lawrence residents who wanted to make damage claims because it couldn't handle the deluge, The Boston Globe reported. The company told residents, many of whom waited for several hours, to come back Monday.
"I am tired. I am frustrated," Renata Rena, who waiting in line with her 3-year-old daughter, told the newspaper.
Joe Hamrock, chief executive of Columbia Gas' parent company, NiSource, said Sunday that the company was taking full responsibility for what happened and is implementing a service restoration plan. He said the devastation and tragedy caused by the explosions are heartbreaking.
"We're in this for the long run," Hamrock said. "We know this has damaged the confidence and trust in our company and what we do. Our full commitment is restoring that trust and that confidence."
Officials said gas company technicians will turn all the meters back after safety inspections of the entire system are complete — a process expected to take several weeks. They warned residents not to turn the meters back on themselves, not to turn on gas appliances until service is restored and to call 911 and leave their homes if they smell gas.