Officials say 11 fires have sparked in places ranging from churches to middle schools since September 18, 2010, in the area known as Old Horsham.
"So far it seems to be dumpsters and outdoor storage facilities, sheds, wood piles, playground equipment. But they've spread, and we've had damage to a school and a home. Somebody's going to get hurt," Horsham Police Sgt. Larry Bozzomo said.
The most recent incident happened Tuesday as homeowner Martin Stout was drifting off to sleep.
"It was a beautiful night, we had our windows open to let the air in. I started to smell smoke. I said to myself, 'This can't be happening again,'" Stout said.
Just a little over a year ago, one of Stout's storage sheds was destroyed by fire.
Tuesday night, its replacement and an adjacent wood pile were targeted.
A second arson spread at the Horsham Montessori School and burned out its library last year. It will reopen soon thanks to donations sparked by the Chamber of Commerce.
"They put out a blurb to all the businesses in Horsham, and we have gotten over 8,000 books," principal Nona Melnick said.
Authorities only have one murky video clip of the presumed arsonist walking away from a fire.
It's not much, but hopefully someone recognizes his walk and calls police.
In the meantime, those targeted by fire are unnerved.
"It is so difficult to go to bed at night not knowing what is going to happen," Stout said.