During the early 1960s, Keyhoe published excerpts from a letter by Hermann Oberth that presented explanations for the flight characteristics of UFOs in terms of gravity control propulsion. Prior to Oberth's letter, Keyhoe had supported arguments for magnetic forces as the source of propulsion for UFOs. The letter caused him to search for the existence of gravity control propulsion research programs. The following is a segment of his findings he had released in his 1966 and 1974 publications: