WASHINGTON: The US military is investigating what appears to have been the first case of American troops killed by a missile fired from a US drone. The investigation is looking into the deaths of a Marine and a Navy medic killed by a Hellfire missile fired from a Predator after they apparently were mistaken for insurgents in southern Afghanistan last week, two senior US defense officials said Tuesday. The officials spoke on condition of anonymity because an investigation is under way. Unmanned aircraft have proved to be powerful weapons in Afghanistan and Iraq, and their use has expanded to new areas and operations each year of those conflicts. Some drones are used for surveillance, and some, such as the drone in this case, are armed and have been used to hunt and attack militants. Officials said this is the first case they know of in which a drone may have been involved in a friendly fire incident in which US troops were killed. They are trying to determine how it happened. Marine Staff Sgt. Jeremy Smith of Arlington, Texas, and Seaman Benjamin D. Rast of Niles, Michigan, were hit while moving toward other Marines who were under fire in Helmand province.Military officials in Afghanistan would provide no details, saying only that it was a friendly fire incident. “A formal investigation will determine the circumstances that led to the incident,” the International Security Assistance Force said in a statement last week. Reports from the field indicate that the Marines who were under attack mistook Smith and Rast for militants heading their way and called in a strike from a US Air Force Predator, one official said.