SLIPPERY ROCK, Pa. (AP) — State police were continuing to investigate the cause of a fire that damaged or destroyed nine recreational vehicles at a western Pennsylvania campground.

The first vehicle was reported on fire just before 4 a.m. Monday at the Moraine Camplands in Brady Township about 4 a.m. Monday.

The fire spread to other vehicle, which were tightly parked together. Unionville Volunteer Fire Chief Nathan Wulff says they were parked only about 10 feet apart.

Nobody was hurt in any of the fires, though two of the campers were known to be occupied at the time.

The member-owned campground opened in 1975. It's located about 40 miles (64.4 kilometers) north of Pittsburgh.