Fire officials say a natural gas leak within the walls of a Yonkers apartment building caused an explosion after a fire broke out. The fire broke out at the Whitney Young Manor Apartments on Nepperhan Avenue, forcing several families out of their homes Monday. Fire officials say cooking grease in a sixth-floor incinerator apparently started the fire. A subsequent explosion from a gas leak buckled the wall on the 10th floor. Two firefighters suffered from minor injuries. Several tenants were treated for smoke inhalation.At least 40 people have been displaced by the fire. The Red Cross is working with building owners to determine the extent of the damage and providing families with temporary lodging.