A San Diego-based Navy corpsman was killed in Afghanistan last week in what appears to be the first case of U.S. troops killed by a missile fired from a U.S. drone.Killed last week in that friendly fire incident in southern Afghanistan were Marine Staff Sgt. Jeremy Smith of Arlington, Tex. and Navy Corpsman Benjamin Rast of Niles, Mich.While Pentagon officials said an investigation is underway, Rast's father Robert said it's an accident which shouldn't have happened."If Ben had been killed by an enemy, it wouldn't make it any easier to cope with. That is an acceptable condition of war," said Robert Rast. "But it's not acceptable for a U.S. Air Force captain to tell an airman to launch a Hellfire missile straight at my son."10News also spoke with a retired Marine lieutenant colonel who said though the incident is tragic, friendly fire events are not unusual."I abhor that but I also understand it," said retired Lt. Col. Thomas Richards. "It's unfortunate but it's not unreasonable for those people to make that mistake under the pressure of combat."The Predator drone and others fly over battlefields operated by remote control from U.S. air bases thousands of miles away. The Predator used by the U.S. Air Force and the one possibly used in the recent friendly fire incident is made by local manufacturer General Atomics.Operators can only see what the drones' cameras show them. According to the Pentagon, in this case, the Predator operator mistook the two men for insurgents who were about to attack a group of known Marines.According to Rasts father, Rast spent about a year in San Diego at Camp Pendleton and at the Naval Medical Center in Balboa Park.Rast's father said his son loved San Diego and loved what he did in the U.S. Navy. In fact, just days before Rast left for Afghanistan, Rast was told he no longer had to go. But Rast's father said Rast wanted to go anyway.Rast's funeral will be held on Saturday in Michigan.