Four homes in Harrisburg were damaged by fire, early Sunday morning.Harrisburg City Fire Chief Brian Enterline says one of the homes, in the 500 block of Radnor Street, had fire on all three floors when crews arrived.The fire spread from porch to porch and window to window, according to Enterline.While crews were fighting the fire, there was an explosion of undetermined origin in the main fire building. According to Enterline, after the explosion, the main fire building partially collapsed.The Chief added, that weather and construction on the rear second floor balconies, made fighting this fire a challenge.The Red Cross is assisting 5 adults and 4 children displaced by the fire.Enterline says a number of firefighters suffered minor injuries, but were treated at the scene and returned to fighting the fire.The chief estimates damage to be in excess of $100,000.The cause of the fire remains under investigation.