A fire that was set intentionally at the old William Penn High School in Harrisburg is under investigation.Witnesses reported seeing young people running from the abandoned building around the time of Tuesday afternoon's fire.The fire chief said the department has responded to fires at the building in the past."It is a large campus. We're on a 25-acre campus. If you walk these hallways, they are longer than you can even see. And to secure a building like this so that it doesn't have vandalism and things like that, it's just nearly impossible," said Chief Brian Enterline.Investigators said they're looking at surveillance video to determine who started the fire.