At first, investigators thought the succession of suspicious fires in abandoned buildings south of Boston might not be connected, because they broke out over such a large swath of suburbs.

But when they kept recovering remnants of fire-starter logs from the scenes, they realized they were dealing with serial arson. That discovery spurred what the state’s fire marshal is calling the largest arson investigation in recent memory, which on Thursday led to prosecutors filing charges against a Middleborough man and his stepson.