Investigators said they have ruled out natural gas as a possible cause of a house fire that killed one person and injured a firefighter Wednesday in Nashua. WMUR-TV reported that investigators dug up and checked gas lines to the house and determined that the fire was not caused by a natural gas problem. The victim’s name has not been released, pending an autopsy. Neighbors reported hearing an explosion or explosions around 2 p.m. Wednesday. Flames spread quickly through the house.