The Italian air force has been ordered to investigate the 1980 crash of a domestic airliner after new reports that it was downed by a missile, the Defense Ministry said. The announcement appeared to be a victory for relatives of the 81 victims aboard the DC-9 who have long alleged that the military conspired to hide evidence about the crash. A 1982 probe concluded that an explosion caused the plane to crash into the sea near Sicily but did not establish the reason for the blast. Now, an investigative team says evidence indicates the airliner was brought down by a missile of undetermined origin. Two television documentaries recently charged that the plane was shot down in error by an Atlantic Alliance fighter.