A $20,000 reward is now being offered for information leading to an arrest and conviction in the Coatesville arsons.
The 18th fire was set Tuesday night and has been ruled a case of arson.
The worst of the fires took place last month.
That fire damaged 15 homes and left dozens homeless.
Federal, state and local officials have set up a task force tip line to help crack the case.
If you have information on the Coatesville arsons, call 484-356-6109.