Columbia Gas of Massachusetts on Sunday said it would “completely replace” the natural gas distribution system in the Merrimack Valley after a series of explosions and fires last Thursday killed one man and damaged or destroyed around 80 structures.

During a press conference, National Transportation Safety Board (NTSB) Chairman Robert Sumwalt said it appeared that a regulator controlling the gas flow between high- and low-pressure parts of the system permitted “significantly greater flows and pressure” into structures in Lawrence, Andover and Lower Andover.

A series of explosions that began around 5 p.m. ET last Thursday rocked the three communities north of Boston, damaging dozens of structures and killing an 18-year-old man after a chimney blew off of a home and crushed the vehicle in which he was sitting. Several people were also injured. The Merrimack Valley system serves about 8,600 customers, who were forced to evacuate.

A preliminary investigation by NTSB determined that the pressure sensors, which monitor the low pressure, were connected to a pipeline that had been cut off and capped.

“If the pressure sensor is connected to a line that’s capped off at both ends…the pressure sensor is going to say ‘not a lot of flow’ and go back and open up the regulator,” Sumwalt said.

Contractors working for Columbia Gas were replacing aging pipeline systems a few blocks from the regulator appearing to have issues, Sumwalt said. Gas upstream from the regulator was flowing at about 75 pounds/square inch (psi), and the pressure is supposed to be reduced to 0.5 psi when gas reaches residential meters, he said.

The pressure issue was traced to Columbus, OH, where the pipeline control room is located for the region’s system, Sumwalt said. The pipeline controller will be interviewed as a part of the investigation.

The NTSB investigation will also focus on the operations of Columbia Gas, including recordkeeping and oversight of contractors. In addition, the investigation will look into records of odor complaints in the area for the past several weeks.

Federal investigators are expected to be on the ground in the Merrimack Valley for at least another week. Authorities warned it could take up to two years to determine the disaster’s exact cause.

The Massachusetts Department of Public Utilities plans to inventory gas appliances in the affected neighborhoods to determine how many have pilot lights that could have ignited the explosions, Sumwalt said.

Columbia Gas of Massachusetts, a unit of NiSource Inc., is developing a plan in collaboration with Gov. Charlie Baker’s office and stakeholders to replace the entire affected 48-mile cast iron and bare steel pipeline system in the three towns. The utility plans to use “state-of-the-art plastic distribution mains and service lines, and modern safety features such as pressure regulation and excess flow valves at each premise.”

“This unprecedented event requires unprecedented action,” said NiSource CEO Joe Hamrock. “With this incident, a life was tragically lost and thousands of other lives were directly affected. We lost the trust of this community and are 100% committed to restoring safety, confidence and peace of mind for everyone in this community. Over time, we hope to earn back the trust we lost during this incident.”

Baker, who was critical of the utility’s initial response to the explosions, used his emergency authority last Friday to put utility Eversource Energy in charge of responding to the emergency. Eversource reported finding another gas leak Saturday near the scene of the fires. It may take weeks for gas service to be restored to all customers.

“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,” Baker said at a news conference Sunday. “We still have a very long way to go,” but he said he was encouraged that people were allowed to return to their homes.

Schools in Andover and North Andover were open Monday, while Lawrence schools remained closed.

Columbia Gas of Massachusetts is in the midst of a multiyear program to modernize its gas distribution system and replace cast iron and bare steel pipeline systems across the state.

“Our commitment to accelerate that work in the Merrimack Valley, while larger in scale than a typical modernization project, is necessary in light of recent events,” NiSource management said. “We remain committed to the modernization of all of our pipeline systems.”

As of early Sunday, residents in the Merrimack Valley had begun to return home. All buildings had been cleared and electric power was restored to virtually all residents by afternoon, NiSource said.

“A full complement of crews remain in the area throughout neighborhoods to assist residents as they return to their homes and answer any questions they may have about gas safety. In addition, NiSource and its insurance carriers are working diligently to process damage claims submitted by customers.”

NiSource is one of the largest fully regulated utility companies in the United States, serving about 3.5 million natural gas customers and 500,000 electric customers across seven states through its local Columbia Gas and NIPSCO brands.