KABUL — Three American civilian contractors and an Afghan national were killed Thursday in a shooting incident at a military base attached to Kabul’s international airport, according to U.S. officials. Suspicion fell immediately on a possible “insider attack” perpetrated by a member of the Afghan security forces, who also have access to the military base at the airport. The sprawling base where the shooting occurred is protected by tall concrete blast walls and filled with hangars, office trailers and maintenance buildings. [...] U.S.