It's back to work for fire investigators from the Yakima County fire marshal's office in Toppenish today as they search for an exact cause to Sunday's fire at Heritage University. Classes were cancelled at noon yesterday because of smoke and water problems. So far no cause has been determined. School officials also closed the childcare center that had no food service. Petrie Hall was historically the school's first building, and housed a cafeteria, kitchen, bookstore, computer labs and several classrooms.