Investigators say a young boy started a weekend fire in Des Moines that caused heavy damage to his house and left his father and brother injured. Des Moines Fire Department spokesman Brian O’Keefe says the boy likely did -not- set the blaze in the home’s vacant bedroom with malice. The six-year-old used a cigarette lighter to set a bed on fire then shut the door and left the house, without waking his family who were all asleep. O’Keefe says it’s not unusual behavior for a child that age to get scared and run away because he doesn’t want to get in trouble. He says the boy’s father was lucky to have awakened and discovered the fast-spreading fire.The father and a 16-year-old son were treated for smoke inhalation. The mother and twin toddlers escaped the fire without injury. O’Keefe says there was a smoke detector in the house but it may not have been working.