On May 9, 2001, Steven M. Greer took the lectern at the National Press Club, in Washington, D.C., in pursuit of the truth about unidentified flying objects. Greer, an emergency-room physician in Virginia and an outspoken ufologist, believed that the government had long withheld from the American people its familiarity with alien visitations. He had founded the Disclosure Project in 1993 in an attempt to penetrate the sanctums of conspiracy. Greer’s reckoning that day featured some twenty speakers. He provided, in support of his claims, a four-hundred-and-ninety-two-page dossier called the “Disclosure Project Briefing Document.” For public officials too busy to absorb such a vast tract of suppressed knowledge, Greer had prepared a ninety-five-page “Executive Summary of the Disclosure Project Briefing Document.” After some throat-clearing, the “Executive Summary” began with “A Brief Summary,” which included a series of bullet points outlining what amounted to the greatest secret in human history.

Over several decades, according to Greer, untold numbers of alien craft had been observed in our planet’s airspace; they were able to reach extreme velocities with no visible means of lift or propulsion, and to perform stunning maneuvers at g-forces that would turn a human pilot to soup. Some of these extraterrestrial spaceships had been “downed, retrieved and studied since at least the 1940s and possibly as early as the 1930s.” Efforts to reverse engineer such extraordinary machines had led to “significant technological breakthroughs in energy generation.” These operations had mostly been classified as “cosmic top secret,” a tier of clearance “thirty-eight levels” above that typically granted to the Commander-in-Chief. Why, Greer asked, had such transformative technologies been hidden for so long? This was obvious. The “social, economic and geo-political order of the world” was at stake.

The idea that aliens had frequented our planet had been circulating among ufologists since the postwar years, when a Polish émigré, George Adamski, claimed to have rendezvoused with a race of kindly, Nordic-looking Venusians who were disturbed by the domestic and interplanetary effects of nuclear-bomb tests. In the summer of 1947, an alien spaceship was said to have crashed near Roswell, New Mexico. Conspiracy theorists believed that vaguely anthropomorphic bodies had been recovered there, and that the crash debris had been entrusted to private military contractors, who raced to unlock alien hardware before the Russians could. (Documents unearthed after the fall of the Soviet Union suggested that the anxiety about an arms race supercharged by alien technology was mutual.) All of this, ufologists claimed, had been covered up by Majestic 12, a clandestine, para-governmental organization convened under executive order by President Truman. President Kennedy was assassinated because he planned to level with Premier Khrushchev; Kennedy had confided in Marilyn Monroe, thereby sealing her fate. Representative Steven Schiff, of New Mexico, spent years trying to get to the bottom of the Roswell incident, only to die of “cancer.”

Greer’s “Executive Summary” was woolly, but discerning readers could find within it answers to many of the most frequently asked questions about U.F.O.s—assuming, as Greer did, that U.F.O.s are helmed by extraterrestrials. Why are they so elusive? Because the aliens are monitoring us. Why? Because they are discomfited by our aspiration to “weaponize space.” Have we shot at them? Yes. Should we shoot at them? No. Really? Yes. Why not? They’re friendly. How do we know? “Obviously, any civilization capable of routine interstellar travel could terminate our civilization in a nanosecond, if that was their intent. That we are still breathing the free air of Earth is abundant testimony to the non-hostile nature of these ET civilizations.” (One obvious question seems not to have occurred to Greer: Why, if these spacecraft are so advanced, do they allegedly crash all the time?)

At the press conference, Greer appeared in thin-framed glasses, a baggy, funereal suit, and a red tie askew in a starched collar. “I know many in the media would like to talk about ‘little green men,’ ” he said. “But, in reality, the subject is laughed at because it is so serious. I have had grown men weep, who are in the Pentagon, who are members of Congress, and who have said to me, ‘What are we going to do?’ Here is what we will do. We will see that this matter is properly disclosed.”

Among the other speakers was Clifford Stone, a retired Army sergeant, who purported to have visited crash sites and seen aliens, both dead and alive. Stone said that he had catalogued fifty-seven species, many of them humanoid. “You have individuals that look very much like you and myself, that could walk among us and you wouldn’t even notice the difference,” he said.

Leslie Kean, an independent investigative journalist and a novice U.F.O. researcher who had worked with Greer, watched the proceedings with unease. She had recently published an article in the Boston Globe about a new omnibus of compelling evidence concerning U.F.O.s, and she couldn’t understand why a speaker would make an unsupported assertion about alien cadavers when he could be talking about hard data. To Kean, the corpus of genuinely baffling reports deserved scientific scrutiny, regardless of how you felt about aliens. “There were some good people at that conference, but some of them were making outrageous, grandiose claims,” Kean told me. “I knew then that I had to walk away.” Greer had hoped that members of the media would cover the event, and they did, with frolicsome derision. He also hoped that Congress would hold hearings. By all accounts, it did not.