William Joe Mitchell, a 46 year old sex offender accused of running off with a 15 year old Florida girl and then releasing her at a Wal-Mart store in the Florida Panhandle after a manhunt last week, has been arrested.  He was detained by Virginia state police at a gas station in Winchester, Virginia.  The girl snuck out of her home on Monday to be with him but told authorities Mitchell had a handgun and said he'd kill her if she drew attention to herself.