This study begins, as the National Security Council requested, by assuming that the September 22, 1979 Vela event was a nuclear detonation. It discusses the possibility that the detonation could have occurred due to an accident, and noted the Defense Intelligence Agency’s suggestion that the Soviet Union might have had reasons to conduct a covert test in violation of its treaty commitments. But most of the study is concerned with other possibilities to explain the incident – a secret test by South Africa or Israel, or India, or Pakistan, or a secret joint test by South Africa and Israel. The 2013 release (which is currently under appeal) includes some information from a “Secret Test by Others” (Pakistan, India) and the map on page 12 that had not been released before.