BOSTON (Reuters) - Thousands of miles of natural gas pipelines in Massachusetts are leak-prone and need repair, utilities have told state regulators, highlighting aging energy infrastructure risks after explosions ripped through three towns outside of Boston this week.

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Federal investigators will examine pipeline maintenance in their probe into the Thursday night blasts in Lawrence, Andover, and North Andover, Robert Sumwalt, chairman of the National Transportation Safety Board, said on Friday. The explosions on a pipeline system belonging to Columbia Gas of Massachusetts destroyed scores of homes, killed one person and injured more than a dozen others in the largest natural gas pipeline incident in nearly a decade.

The investigation could provide insight into the nation’s aging pipeline infrastructure. Roughly half of the 2.4 million miles of pipelines crisscrossing the United States were installed before 1970, said Deborah Hersman, chief executive of the non-profit National Safety Council and a former chairman of the National Transportation Safety Board.

Columbia Gas, a unit of NiSource Inc NI.N, owns and operates nearly 5,000 miles of gas pipeline across Massachusetts. But about 15 percent of that includes leak-prone pipes, the company told the Massachusetts Department of Public Utilities in April, as part of its gas system enhancement plan filing.

Columbia Gas and other utilities say the Massachusetts natural gas distribution system is one of the oldest in the United States, with sections built more than a century ago. The company told regulators that it repairs more than 1,200 leaks a year, on average, mostly on its main line pipes.

In 2016, for example, replacing leak-prone pipes was expected to cost it about $56 million.

Columbia had told customers it was conducting upgrades on parts of its system shortly before the explosions.

The company was not available for comment.

Columbia Gas has had a clear safety record for more than a decade in Massachusetts, though a pipeline rupture in West Virginia in 2012 triggered an explosion that destroyed three houses, according to records from the National Transportation and Safety Board and the Pipeline and Hazardous Materials Safety Administration.

Other Massachusetts utilities are also racing to make upgrades.

Eversource Energy ES.N has disclosed that about one-third of its 3,3000 miles of distribution pipeline is leak-prone. The utility replaced about 40 miles of such pipes in 2017 and had plans for another 45 miles this year, according to disclosures with state regulators in April.

Boston Gas, a unit of National Grid, replaced 107 miles of leak-prone pipes in 2016, disclosures to regulators show. But the utility has said it does not expect to have all of leak-prone pipes replaced until 2039.

Democratic Senators Ed Markey and Elizabeth Warren urged a Congressional investigation into the explosions to prevent any recurrence, according to a joint statement released on Friday.

Investigators suspected “over-pressurization of a gas main” was a factor in the incident.