[2] Contemporary media summarized four topics discussed in the book:[3] In 1960, Ruppelt authored a second edition in which he reported being "positive" that UFOs do not exist.
In the foreword, Ruppelt argues: "It is well known that ever since the first flying saucer was reported in June 1947 the Air Force has officially said that there is no proof that such a thing as an interplanetary spaceship exists.
[6][7] Ruppelt begins Chapter One with a tale, set in Summer 1952, where an unnamed F-86 fighter pilot fires on an unidentified target, only to be ridiculed as having 'cracked up' or panicked.
After initial skepticism, sightings from military personnel caught the attention of the Air Force Technical Intelligence Center.
[9] Ruppelt explains that "as more UFO's were observed near the Air Force's Muroc Test Center, the Army's White Sands Proving Ground, and atomic bomb plants, ATIC's efforts became more concentrated."
[12] The Federal Bureau of Investigation (FBI) declared those documents to be "completely bogus", and many ufologists consider them to be an elaborate hoax.
[14] Ruppelt recalls "They might be common meteorites, psychologically enlarged flares, or true UFO's, but whatever they were they were playing around in one of the most sensitive security areas in the United States.
[14] In Summer 1949, the Air Force's Cambridge Research Laboratories established "Project Twinkle" to attempt to photograph the objects; It yields no results.
[15] Folklore historian Curtis Peebles described the Estimate as "the most controversial document in the early history of the flying saucer myth".
[16] No copy of this document or any other corroboration of Ruppelt's claim has been produced,[17] and Popular Mechanics called the report "probably more mythological than real".
Chapter Seven, "The Pentagon Rumbles", discussed the publication of Frank Scully's book Behind the Flying Saucers with its tales of the Aztec, NM UFO hoax.
The Los Alamos-Albuquerque area, Oak Ridge, and White Sands Proving Ground rated high.
[24] Chapter Twelve, "The Washington Merry-Go-Round", describes official reactions to the D.C. incident, culminating in Gen. Sanford's press conference.
", describes Ruppelt's investigation of a Boy Scout leader alleged burned and rendered unconscious by a UFO.
[26] In Chapter Fourteen, "Digesting the Data", Ruppelt describes plans to observe UFOs during the H-Bomb tests in the Pacific and a proposal for 'visual spotting stations' in northern New Mexico.
", Ruppelt opines "I wouldn't want to hazard a guess as to what the final outcome of the UFO investigation will be, but I am sure that within a few years there will be a proven answer.
Air Force Won't Deny It", writing "Ruppelt reports their conclusion as one of rejection the theory that the objects are space ships, on the ground that there is no definite proof.
[35] In the 1970s, UFO researcher J. Allen Hynek suggested that Ruppelt's "book should be required reading for anyone seriously interested in the history of this subject".