A gas company announced Tuesday that it had reached a settlement with three communities in the Merrimack Valley of Massachusetts after explosions and fires ripped through dozens of homes there last year, killing one person and injuring several more.

The blasts on Sept. 13 forced thousands of people to evacuate. Residents of the communities affected — Lawrence, Andover and North Andover — were not immediately sure what was to blame.

The authorities investigating the episode soon focused their attention on Columbia Gas of Massachusetts, which supplies natural gas to the area and had been working upgrade the distribution system around the time of the explosions.

On Tuesday, about eight months after the explosions, Columbia Gas, whose parent company is NiSource, announced a settlement with the municipalities: It is paying $80 million to cover costs associated with infrastructure repair and other expenses.