Half a century after the Warren Commission concluded there was no conspiracy in John F. Kennedy’s assassination, the commission’s chief conspiracy hunter believes the investigation was the victim of a “massive cover-up” to hide evidence that might have shown that Lee Harvey Oswald was in fact part of a conspiracy. In new, exclusive material published today in the paperback edition of a bestselling history of the investigation, retired law professor David Slawson tells how he came to the conclusion, on the basis of long-secret documents and witness statements, that the commission might have gotten it wrong....

Slawson’s most startling conclusion: He now believes that other people probably knew about Oswald’s plans to kill the president and encouraged him, raising the possibility that there was a conspiracy in Kennedy’s death—at least according to the common legal definition of the word conspiracy, which requires simply that at least two people plot to do wrongdoing. “I now know that Oswald was almost certainly not a lone wolf,” Slawson says.

Slawson is not describing the sort of elaborate, far-fetched assassination plot that most conspiracy theorists like to claim occurred, with a roster of suspects including the Mafia, Texas oilmen, anti-Castro Cuban exiles, southern segregationists, elements of the CIA and FBI, and even President Johnson. Slawson did not believe in 1964, and does not believe now, that Fidel Castro or the leaders of the Soviet Union or of any other foreign government were involved in the president’s murder. And he is certain that Oswald was the only gunman in Dealey Plaza.

What Slawson does suspect is that Oswald, during a long-mysterious trip to Mexico City only weeks before the assassination, encountered Cuban diplomats and Mexican civilians who were supporters of Castro’s revolution and who urged Oswald to kill the American president if he had the chance. “I think it’s very likely that people in Mexico encouraged him to do this,” Slawson told me. “And if they later came to the United States, they could have been prosecuted under American law as accessories” in the conspiracy.