Perhaps sensing this, the publishers of Brothers in Arms claim that it depicts the events surrounding the assassination “in a narrative non-fiction format, using … techniques of the most highly readable novels.” Written by veteran investigative reporter Gus Russo in collaboration with novelist and screenwriter Stephen Molton, this elegantly composed book depicts those few seconds in Dallas as being the culmination of a blood feud between two sets of brothers, the Castros and Kennedys. While Fidel and John were the figureheads in this war, it is really Raúl Castro and especially Bobby Kennedy—the “altar boy [who] had grown up to become a Savonarola in a rumpled Brooks Brothers suit”—who were its field marshals. Indeed, it’s R.F.K. who comes across as the truly tragic figure here, bearing significant responsibility for his brother’s death, yet prevented from avenging it by world-historical events beyond his control.

For Messrs. Russo and Molton, the trail to Dealey Plaza begins in Minsk, where Lee Harvey Oswald washed up after defecting to the Soviet Union in 1959. There, he came to the attention of the Cubans, who had spies training in the city. After Oswald’s repatriation to Texas in 1962, Cuban intelligence, led by Raúl, kept tabs on this odd character, supporting him with small sums of money and subtle encouragement. Not that he was ever an actual operative, the authors maintain, since “he was neither smart enough nor mentally reliable enough to be trusted. What he was, within weeks of his arrival, was a Cuban-aligned sleeper agent, a potential asset who might prove useful to Havana one day.”

According to this theory, Oswald’s increasingly erratic behavior stemmed from his desire to “prove his devotion to Fidel’s cause.” His attempted assassination of the right-wing firebrand General Edwin Walker, his flirtation with anti-Cuban groups in New Orleans and, ultimately, his plan to kill J.F.K. were all part of an effort to earn him a place in Cuba’s revolution. He finally took his dossier to Mexico City in late September of ’63, where he presented it at the Cuban embassy. Soon, the authors maintain, he was meeting with high-ranking Cuban intelligence officials and even having an affair with a beautiful embassy employee. By the time he returned to Texas, the Cubans had decided to egg him on to kill Kennedy with false promises of sanctuary.

But why would Castro risk provoking the lethal ire of a superpower that had already attempted to crush his government? Messrs. Russo and Molton argue that Cubans’ gambit was a reaction to intelligence that the Kennedy brothers, Bobby in particular, were planning to reprise the Bay of Pigs operation with another attempt to overthrow Castro’s regime, in late 1963. By playing Oswald, the joker in their pack, the Cubans not only forestalled the coup, but also insulated themselves against payback from L.B.J., who knew that blaming Havana would necessitate a retaliation that would involve the Russians and risk a nuclear holocaust. As for Bobby, he had an added incentive to bury the plot, since pursuing “a Cuban angle would expose the White House’s own murder plotting, marring Jack’s name for all time.”

There’s something deeply intriguing about the picture the authors paint of the beleaguered Cubans taking an interest in this bizarre young man who showed up on their doorstep, then letting him run to see what kind of mischief he could wreak. It’s only when Messrs. Russo and Molton stray beyond the realm of plausible fiction by trying to establish a factual record of active Cuban support for Oswald on the ground in Dallas that their story loses its mythic power.

LEGACY OF SECRECY proves, in many ways, to be a mirror image of Brothers in Arms. Written by veteran Kennedy investigator Lamar Waldron (assisted by Air America host Thom Hartmann), it asserts that Oswald was in fact an anti-Castro agent involved in a C.I.A. plot to overthrow the Cuban leader, which was slated to take place just days after Kennedy’s killing. “Oswald’s trip to Mexico City in September 1963 was probably his attempt to enter Cuba,” the authors assert, “as one of the assets the CIA was tasked with getting into Cuba for the upcoming … coup plan.” What the C.I.A. didn’t know was that elements of the Mafia, who had been used in earlier schemes to knock off Castro, had infiltrated the plot and turned it against Kennedy. Their motive was to strike back at Bobby, the notoriously anti-Mafia attorney general, as well as to get their casinos back into Cuba, something J.F.K. opposed.

Like Messrs. Russo and Molton’s Cubans, the Mafia were emboldened by the knowledge that Bobby could not come after them, since to do so would expose his brother’s illegal plotting against Castro, as well as risk a confrontation with the Soviet Union. “As a result, Robert Kennedy and other high officials had to withhold key information in order to prevent, in the words of President Johnson, a nuclear holocaust that could cost the lives of ‘forty million Americans.’”

Although Legacy of Secrecy is exhaustively researched, the new “conclusive evidence” it boasts of presenting often proves to be less than convincing. For instance, the confession of the plot’s alleged mastermind, New Orleans crime boss Carlos Marcello, is nothing more than a single bit of hearsay reported by a prison informant. Similarly, the new “proof” of C.I.A. involvement comes during a drunken anti-J.F.K. tirade by the operations chief at its Miami station, David Morales, in which he declares, “We took care of that son of a bitch, didn’t we?”

Even more tenuous are the authors’ attempts to implicate the Mafia conspirators in the killings of Martin Luther King and Bobby Kennedy. In the case of King, they claim that a Georgia racist named Joseph Milteer, who was allegedly a minor player in the J.F.K. plot, asked Marcello to provide a hit man to kill the civil rights leader. The don, a fellow racist, obliged by offering a small-time drug-runner named James Earl Ray. As for Bobby, the authors strain the reader’s credulity to the breaking point when they posit very flimsy evidence that R.F.K.’s killer, Sirhan Sirhan, was in some way connected through his lawyer to mobsters behind the killing of both J.F.K. and King.

FOR ITS PART, JAMES W. Douglass’ JFK and the Unspeakable puts forward a scenario in which J.F.K. was cut down by a C.I.A.-administered plot that used Oswald as a patsy. Mr. Douglass, “a longtime peace activist,” believes Kennedy was murdered by shadowy forces because he was in the process of renouncing the cold war and all its toxic legacies: the arms race, Vietnam, the anti-Castro campaign, even predatory capitalism in the form of Big Steel. Using language borrowed from Thomas Merton, Mr. Douglass describes the plot as being informed by a vague force of evil “whose depth and deceit seemed to go beyond the capacity of words to describe.” Just who represents this unspeakable force remains unspecified. “We have no evidence as to who in the military-industrial complex may have been given the order to assassinate President Kennedy. That the order was carried out by the Central Intelligence Agency is obvious. The CIA’s fingerprints are all over the crime and the events leading up to it.”

To support this theory, the author deploys a selective rehashing of such conspiracy chestnuts as the Altgens photo, which allegedly shows Oswald standing in the doorway of the Texas Book Depository at the exact moment he should have been firing away on the sixth floor. The book’s real interest lies in its portrait of J.F.K. as a man who, after the Bay of Pigs and the near-apocalypse of the Cuban missile crisis, was in the process of turning away from the saber rattling of the early days of his presidency. Dissenters will point to his authorization of a second action against Castro and the deadly 1963 coup against Vietnamese president Diem as evidence that the J.F.K. who went to Dallas was the same old cold warrior. Mr. Douglass, however, does make a convincing case that J.F.K. was becoming deeply disillusioned with the bellicosity of American foreign policy and the inordinate power of the military-industrial complex.

Whether this got him killed remains, like just about everything else that happened in Dallas, the stuff of myth.

Stephen Amidon’s new novel, Security, will be published by Farrar, Straus and Giroux in February. He can be reached at books@observer.com.