Columbia Gas of Massachusetts and parent NiSource Inc. on Monday agreed to settle class action lawsuits triggered by a natural gas pipeline explosion last September that rocked three Merrimack Valley towns and killed one man.

In addition to the one fatality, more than two dozen people were injured and 131 structures were damaged or destroyed in the explosions and fires in Lawrence, Andover and North Andover.

The incidents, which led to a mass evacuation from the three communities, may have been caused in part by gaps in construction work orders for the old system prepared by the NiSource Massachusetts subsidiary, according to the National Transportation Safety Board.

“As part of the approximately $1 billion in funds dedicated to address the needs of affected customers, residents and communities, the company will pay $143 million into a settlement fund for a class of plaintiffs, per the terms of the class action settlement,” NiSource officials said. “The class of plaintiffs includes thousands of residents and businesses affected by the incident.”

The settlement followed a voluntary mediation process, covering multiple class action lawsuits, in which all parties engaged in negotiations over several months.

"What happened last September was tragic, and we will always be mindful of its impact on our customers and everyone in the communities we serve, including those represented by this settlement," said NiSource CEO Joe Hamrock. The settlement “marks another important step forward, as we continue to fulfill our commitment to residents and businesses.”

Specifically, NiSource and the proposed class of plaintiffs entered into an agreement in principle (AIP) to resolve all class action litigation related to the gas event. The AIP is subject to court approval.

“The ongoing Columbia Gas-managed claims process, which was established immediately after the incident, is still processing claims,” NiSource noted. “It will continue to do so, with oversight of the mediator, until the court gives preliminary approval to the settlement, at which time a settlement claims administrator will take over the process.”

All residents and businesses in Andover, Lawrence, and North Andover are eligible to file claims through the process, with more information to be provided in the coming weeks, according to NiSource.

The  $1 billion in funds dedicated to address the needs of the affected customers, residents and communities includes the $143 million settlement, as well as settlements with individual families;  restoration work conducted in homes and businesses; temporary housing needs for affected residents; and funds paid to residents, businesses and affected municipalities through the claims process to date.

NiSource noted that it also has dedicated funds for replacing pipeline in the region. The total costs of the recovery related to the Merrimack Valley incident are to be updated in conjunction with the company’s second quarter earnings report.

Columbia Gas of Massachusetts serves more than 320,000 gas customers in the southeastern part of the state, the greater Springfield area and the Merrimack Valley.