The National Transportation Safety Board has determined overpressurized lines and inadequate fail-safes led to last year's gas explosions in three Merrimack Valley communities.

The NTSB final report, released Thursday, said Columbia Gas of Massachusetts used "inadequate" and "deficient" procedures leading up to and in the aftermath of gas explosions that erupted in Lawrence, Andover and North Andover.

At the time, Columbia Gas — the utility that owns the natural gas infrastructure in the area — used only sensing lines and regulators to detect and prevent overpressurization. The report said data collected over the past 50 years on similar accidents shows the use of those two methods alone "are not optimal to prevent" such accidents.

In August, Columbia Gas president and COO Mark Kempic told WBUR the utility was almost done installing automatic shutoff devices.

"It's hard to speculate on what could have been," Kempic said at the time when asked if certain fail-safes would have prevented the explosions. "It was an unprecedented event in the history of our company [and an] unprecedented event in the history of the natural gas industry."

But the final NTSB report explicitly states:

. . . had NiSource adequately performed system engineering management throughout the project work, the safety risk of an overpressurization likely would have been identified, along with appropriate mitigations implemented before undertaking the construction activities.

In a statement released Wednesday, Columbia Gas said its own assessment "generally aigns" with the NTSB's and that the company has taken steps to prevent a similar failure from happening again.

"We look forward to reading the final report and welcome it because it will help us, our industry partners, the public, and others learn from this tragedy," the statement read. "As we've said since that tragic day, we take responsibility for what happened."

The report indicates NiSource, the parent company of Columbia Gas, has implemented the recommendations to prevent a similar event from happening again, including:

Making sure all records and documentation of the natural gas systems are traceable.

Making sure all applicable departments review construction documents for accuracy, completeness and correctness.

Having the ability to identify system threats.

Implement procedures during work on gas mains to limit risks--continually monitoring gas main pressures and having the ability to immediately shut down the system.

The report also urges 31 states to require licensed professional engineer approval and stamping for all natural gas infrastructure projects.

One person was killed in the explosions in northern Massachusetts last year, 22 people were hospitalized and 131 structures were damaged.