RAYNHAM — The charred sites began appearing across Southeastern Massachusetts in late September, a growing cluster of at least 16 suspicious fires from Quincy to Sandwich that is feared to be the destructive work of a serial arsonist.

The latest fire, ruled an ­arson, happened late Monday night in a vacant outbuilding at the old Raynham dog track, ­only one day after another unexplained fire erupted at a building in Weymouth.