A Marine and an Air Force reservist based in California were killed in separate accidents, officials said. The Marine, Pfc. Christopher J. Bellecci, 22, of Vallejo, was killed at the Marine Corps Air-Ground Combat Center when a 20-ton amphibian tractor backed over him in the dark, military authorities said. He was on the ground in a sleeping bag on a training mission at the combat center 125 miles northeast of Los Angeles when the vehicle backed over his abdomen. Air Force reservist Master Sgt. James L. Borland, 47, of Moreno Valley, was killed in a freak accident when a window burst on a tanker aircraft flying over the Atlantic Ocean, causing the cockpit to depressurize, military officials said. None of the 17 others aboard the plane were hurt.