Investigators ruled out sabotage or a discarded cigarette as the cause of an Air Canada DC-9 fire last Thursday. Investigators concentrated instead on a known electrical problem in a rear restroom that was noted about the same time as the outbreak of fire and smoke. The airplane, flying from Dallas to Toronto, caught fire in midair. With help from air controllers, the pilot was able to make an emergency landing at Greater Cincinnati Airport. Of the 46 on board, 23 survived.