A state police fire marshal was investigating a fire that heavily damaged a vacant, family-owned glass plant in southeastern Pennsylvania.Authorities say about 100 firefighters battled the blaze at the Penn Glass plant just south of Lancaster. The fire was reported about 11:30 p.m. Saturday. Nobody was found inside the building and authorities have not  confirm a report that a door to the factory had been broken. The family-owned business closed last fall and the building had been for sale since December.