Luis Elizondo made headlines in 2017 when he resigned as a senior intelligence official running a shadowy Pentagon program investigating U.F.O.s and publicly denounced the excessive secrecy, lack of resources and internal opposition that he said were thwarting the effort. Now Elizondo, 52, has gone further in a new memoir. In the book he asserted that a decades-long U.F.O. crash retrieval program has been operating as a supersecret umbrella group made up of government officials working with defense and aerospace contractors. Over the years, he wrote, technology and biological remains of nonhuman origin have been retrieved from these crashes. "Humanity is, in fact, not the only intelligent life in the universe, and not the alpha species," Elizondo wrote. The book, "Imminent: Inside the Pentagon's Hunt for U.F.O.s," is being published ... after a yearlong security review by the Pentagon. In a foreword to the book, Christopher Mellon, a former deputy assistant secretary of defense for intelligence, wrote that, without Elizondo, "the U.S. government would still be denying the existence of U.A.P. and failing to investigate a phenomenon that may well be the greatest discovery in human history." The program led by Elizondo investigated sightings, near-misses and other encounters between U.A.P. and Navy jets. It also collected data from incidents involving military and intelligence operations, including images of extraordinary craft maneuvers.