The series of fires and explosions last week in communities north of Boston may be “linked to an overpressurization of the gas distribution system” used by Columbia Gas, Massachusetts’ two senators said in a letter to Columbia’s president and the chief executive of its parent company, NiSource. Federal regulators, the lawmakers said, determined that the pressure was 12 times higher than it was supposed to be.

The fires and explosions led to mass evacuations and several injuries in the towns of Andover, North Andover, and Lawrence, as well as one death.

The chairman of the National Transportation Safety Board said over the weekend that a spike in pressure in one of the pipelines was detected in Columbus, Ohio, at a NiSource facility. Before the fires, Columbia had said it was upgrading its natural gas lines in the area, but it’s unknown if those upgrades contributed at all to the pressure spike or subsequent disaster.

About 8,600 of Columbia’s customers were affected by the outages and had to have their gas shut off and equipment inspected, the company said last week, while more than 18,000 homes had their electricity cut off at some point. Residents who were forced to evacuate started to return over the weekend, although many still don’t have gas service.

A class action was filed on behalf of residents who were forced to evacuate, the Associated Press reported. “These individuals have had their lives turned upside down,” one of the attorneys involved in the case told the AP.