Sign was officially inconclusive regarding the cause of the sightings. However, according to US Air Force Captain Edward J. Ruppelt (the first director of Project Blue Book), Sign's initial intelligence estimate (the so-called Estimate of the Situation) written in the late summer of 1948, concluded that the flying saucers were real craft, were not made by either the Soviet Union or United States, and were likely extraterrestrial in origin. (See also extraterrestrial hypothesis.) This was subsequently rejected by Gen. Hoyt Vandenberg, USAF Chief of Staff, citing a lack of physical proof. Vandenberg subsequently dismantled Project Sign.