FRESNO, Calif. (KSEE/KGPE) - An investigation is underway after the Fresno Fire Department said they found construction materials and a porta potty on fire at the Manchester Mall Tuesday night. Fire crews say they responded to a structure fire at the Manchester Mall. Upon arrival, they found a porta potty and other construction materials burning with the fire spreading to the building. Firefighters say Fire Station 5 was about a half mile away and they prevented the fire from spreading and causing significant damage to the structure. The cause of the fire, however, remains unknown at this time.