DEALEY PLAZA, NOV. 22, 1963: The roar of the crowd ... the flicker of an 8-millimeter home-movie camera ... the wave, followed by the crack of gunfire ... the splatter of blood and brain matter. The murder of John F. Kennedy reruns in the nation's collective consciousness like a horror show that never closes. Fully 7 out of 10 Americans think a nameless, craftily concealed conspiracy did Kennedy in -- and why wouldn't they? For three decades, harum-scarum conspiracy theories have come not as single spies but in battalions, marching at us out of 200 books and a Hollywood blockbuster. Saturnine superpatriots, bearded Marxists, vengeful Mafiosi, power-mad bureaucrats, ticked-off generals, burnt-out spooks -- the suspects stretch to the horizon. Ten new assassination books arrive this fall on the eve of the slaying's 30th anniversary: 10 books with a smell about them, including one with the smell of truth.

That book is Gerald Posner's ''Case Closed," to be published September 1 by Random House (excerpts start on Page 74). A 39-year-old lawyer turned writer (his credits include co-authoring the acclaimed biography ''Mengele: The Complete Story," 1986), Posner achieves the unprecedented. He sweeps away decades of polemical smoke, layer by layer, and builds an unshakable case against JFK's killer. To do this, he had to fully reappraise a massive evidentiary record, plunging in without a clue as to where or when he would come out. He reindexed all 26 volumes of Warren Commission testimony and the 1979 report of the House Select Committee on Assassinations and cross-referenced material in hundreds of books and articles. Then, to fill gaps not bridged by his labyrinth of 3-by-5 cards, he did more than 200 interviews of his own. His conclusion: Yes, Lee Harvey Oswald -- the pathetic ''patsy" of so many conspiracy scenarios, the putative fall guy of the much maligned Warren Commission -- gunned down Kennedy. And yes, he acted alone.

Americans determined to disbelieve this thesis may be beyond persuasion. But readers who follow Posner's analysis with an open mind will have their eyes opened in new ways. The Warren Commission correctly identified Oswald as the killer but filed a brief against him that was hobbled by mistakes and unanswered questions. Posner now performs the historic office of correcting the mistakes and laying the questions to rest with impressive finality, bringing the total weight of evidence into focus more sharply than anyone has done before.

Shell proof. The central issue raised by the physical evidence has always been whether a single bullet could pass through Kennedy's upper back and also cause the wounds suffered by Texas Gov. John Connally. The two men were struck almost simultaneously. If a different bullet hit Connally, only a second gunman could have fired it. Posner demonstrates how computerized re-enactments, special enhancements of the Zapruder film, new bullet-impact tests and medical expertise have at last proved the single-bullet theory beyond a reasonable doubt (see Page 88). ''The chapter on the single bullet is a tour de force, absolutely brilliant, absolutely convincing," says Stephen Ambrose, the distinguished biographer of Dwight Eisenhower and Richard Nixon, who was previously a strong single-bullet skeptic.

''Case Closed" demolishes just as decisively another cause celebre among conspiracy theorists: contradictions between how the treating physicians at Parkland Hospital and the autopsy doctorsdescribed Kennedy's wounds. Exit wound or entrance wound? Big or small? High or low? Warren Commission critics have treated the discrepancies as proof that the body was tampered with to obscure the presence of a gunman or gunmen who fired from the grassy knoll, on Kennedy's right, while Oswald (or someone else) fired from the Texas School Book Depository behind the president. But when Posner himself interviewed the Parkland doctors, all but one agreed with the autopsy findings, conceding that their original observations, made hastily under great stress, had been incomplete, partially incorrect or subsequently distorted by conspiracy writers.

Conspiracists have also long believed that if they could get a look inside Oswald's KGB file, it might well show he was a Soviet agent. ''Case Closed" not only examines that file but reports the author's interview with Yuri Nosenko -- the first ever exclusively devoted to the former KGB officer's supervision of the Oswald case. Both confirm that Oswald was not only not an agent but was deemed totally untrustworthy by the spy agency.

Previously undisclosed files cited by Posner also play havoc with the romanticized portrait of New Orleans District Attorney Jim Garrison that director Oliver Stone presents in his 1991 film, ''JFK." The files -- of Garrison's own investigators -- buttress an earlier generation's verdict that Garrison's chaotic assassination prosecution twisted the truth with cynical abandon. On issue after issue, Posner catches Stone and the major conspiracy writers in serious misrepresentations of the evidence

THE GRASSY-KNOLL FOLLIES

The high quotients of common sense, logic and scrupulous documentation found in ''Case Closed" are niceties not often found in the field of assassination studies. One book entitled ''Is President John F. Kennedy Alive -- and Well?" has run through 15 editions. Another book, ''Best Evidence," made the bestseller lists in 1981 with its theory that on the flight back to Washington, Kennedy's body was stolen from his casket right from under the eyes of the first lady and presidential aides and surgically altered to disguise wounds made by a second gunman. The author, David Lifton, who is at work on yet another conspiracy book, has spent his adult life trying to unmask the JFK plotters. In the mid-1960 she did photo enhancements of shrubbery on the grassy knoll and thought he could discern a man with a periscope, a man with a machine gun, another with an electronic headset, still another wearing a Kaiser Wilhelm helmet, and a galoot who was either Douglas MacArthur or the general's dead ringer.

The conspiracy writers are fed by a network of amateur sleuths who keep vast files of clippings in their basements and troll for fresh witnesses who all too often have found their tongues after half a lifetime of terrified silence. By one count, 30 men have been identified by buffs as ''the second gunman" or have themselves ''confessed" to firing shots at Kennedy, usually from the grassy knoll.

The quest for Kennedy's killers long ago became the domain of both hobbyists and profiteers. A for-profit JFK Assassination Information Center prospers in Dallas by selling bumper stickers, T-shirts and other murder memorabilia and charging people $ 4 to view its exhibits. Conspiracy buffs meet for three days each year in Dallas ($ 150 registration fee) to swap theories, attend seminars on such topics as ''Media Coverup -- Then and Now" and welcome star conspiracy ''witnesses," who sign autographs like rock stars. Conspiracy ''research" occasionally forms a symbiotic alliance with tourism. Sponsors of a three-day John F. Kennedy Assassination Symposium last week at Laurentian University in Sudbury, Ontario, advertised that participants could also wrap in a visit to the Shakespeare festival and take their children to a local amusement park and an underground nickel mine.

Assassination artifacts can be big moneymakers. Jack Ruby's gun went for $ 200,000 at auction last year; the new owner offers 5,000 ''limited edition" bullets shot from it for $ 500 each. Character assassination of the dead and group libel can be even more lucrative. Warner Bros.' ''JFK," a heavily fictionalized film tarring Earl Warren, Lyndon Johnson, the CIA and the FBI, has grossed $ 196.5 million worldwide.

Culture shock. Every presidential assassination and every war in U.S. history has spawned suggestions of secret plots and hidden agendas, but never before have the conspiracy alarms sounded so loud so long. Jack Ruby's shooting of Oswald on live television only 48 hours after his arrest (box, below) stirred visceral suspicions that were only deepened by the troubled epoch that followed -- the calamity of Vietnam, the social turmoil of the late '60s, Watergate and the disillusionments of the '70s. JFK's murder came to be remembered as a loss of national innocence, which served to magnify the appeal of conspiracy scenarios. A single individual might murder a man, but a whole era? Many people want a more formidable set of villains. ''If you put 6 million dead Jews on one side of a scale and on the other side put the Nazi regime ... you have a rough balance: greatest crime, greatest criminals," historian William Manchester has written. ''But if you put the murdered president of the United States on one side of a scale and that wretched waif Oswald on the other side, it doesn't balance. You want to add something weightier to Oswald. It would invest the president's death with meaning, endowing him with martyrdom. He would have died for something. A conspiracy would, of course, do the job nicely."

Some conspiracists are so deep in denial -- the actual evidence notwithstanding -- that they have diminished Oswald's role to that of a fall guy without a drop of blood on his trigger finger. In the film ''JFK," Oswald loiters harmlessly in the Texas School Book Depository lunchroom while Kennedy is killed by operatives of a vast military-industrial conspiracy bent on preventing him from pulling U.S. forces out of Vietnam (never mind that most historians agree that cold warrior Kennedy had revealed no such intent). Posner puts Oswald back at the center of the action by establishing who he really was and what he really did November 22, 1963 .