Capitalizing on the interest, Keyhoe expanded the article into a book, "The Flying Saucers Are Real" (1950); it sold over half a million copies in paperback. He argued that the Air Force knew that flying saucers were extraterrestrial, but downplayed the reports to avoid public panic. In Keyhoe's view, the aliens — wherever their origins or intentions — did not seem hostile, and had likely been surveilling the earth for two hundred years or more, though Keyhoe wrote that their "observation suddenly increased in 1947, following the series of A-bomb explosions in 1945." Dr. Michael D. Swords characterized the book as "a rather sensational but accurate account of the matter." (Swords, p. 100) Boucher and McComas praised it as "cogent, intelligent and persuasive."