BOSTON (CBS/AP) — Federal investigators’ final report on last year’s Merrimack Valley gas explosions said the probable cause was Columbia Gas’ “weak engineering management.”

A teenager died, dozens of other people were injured and more than 100 buildings were destroyed or damaged in the Sept. 13, 2018 disaster. Thousands of homes and businesses in Lawrence, Andover and North Andover were also left without natural gas service for heat and hot water for months in some cases.

Watch: The Merrimack Valley Explosions, One Year Later

Investigators concluded in October 2018 that the series of explosions and fires had been caused by a gas line that became overpressurized during a routine replacement of old cast iron pipelines in Lawrence.

The National Transportation Safety Board, which investigates major pipeline accidents, said Columbia Gas should have been aware of the inherent risks to the upgrade work and taken proper precautions.

In its final report released Thursday, the agency said the probable cause was “was Columbia Gas of Massachusetts’ weak engineering management that did not adequately plan, review, sequence, and oversee the construction project that led to the abandonment of a cast iron main without first relocating regulator sensing lines to the new polyethylene main.”

The NTSB added that another part of the problem was “a low-pressure natural gas distribution system designed and operated without adequate overpressure protection.”

Lawrence Mayor Dan Rivera said in a statement Thursday that he believes the report is enough evidence to revoke Columbia Gas’s licence to distribute gas in Massachusetts.

“The removal of their license is, in part, to punish Columbia Gas, but more importantly, sets a standard for utilities to place community safety above all else,” Rivera said. “Short of the removal of their license, the culture that comes through in this report, that bred this disaster, will not change.”

Columbia Gas’ parent company, NiSource, said in a statement Thursday it will review the NTSB report and “based on lessons learned, we have taken a series of steps to prevent something similar from happening again.”

Earlier this month, a judge gave preliminary approval to a $143 million settlement in a lawsuit brought against Columbia Gas. The proposal to distribute proceeds calls for six categories of lump sum payouts, ranging from up to $50 for a “nominal” disruption to up to $15,000 for a “major” disruption.

A final approval hearing is scheduled for Feb. 27.

(© Copyright 2019 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)

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