Philip Shenon, a former Washington and foreign correspondent for the New York Times, is author, most recently, of A Cruel and Shocking Act: The Secret History of the Kennedy Assassination. This essay is drawn from the afterword to the new paperback edition of the book, scheduled for publication by Picador on Feb. 3.

H alf 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.

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Fifty-one years ago this winter, working from a cramped, paper-strewn temporary office on Capitol Hill, a fresh-faced 33-year-old Denver lawyer named David Slawson was earning his place in modern American history.

It was President John F. Kennedy’s assassination that brought Slawson to Washington. In January 1964, two months after JFK’s murder in Dallas, Slawson was part of a small group of hotshot young lawyers recruited to the capital to join the hastily organized staff of the Warren Commission, the panel convened by President Lyndon B. Johnson to investigate his predecessor’s death.

The lawyers, most only a few years out of law school, would do the bulk of the commission’s detective work in determining how and why the president had been killed. And the Harvard-educated Slawson, in particular, had an extraordinary assignment on the staff. Although he had no background in foreign affairs or law enforcement, he was responsible—at times, single-handedly—for the search for evidence of a foreign conspiracy in the assassination. When the commission issued a final report, in September 1964, that identified Lee Harvey Oswald as the sole assassin and effectively ruled out any conspiracy, foreign or domestic, Slawson was satisfied. “I was convinced—then—that we had it right,” he told me last year.

For most of the next five decades, Slawson, who went on to a distinguished teaching career at the law school at the University of Southern California, tried to put his work on the commission behind him, even as the national debate about the Kennedy assassination and the legacy of the Warren Commission continued to rage. He was content mostly to keep his silence, continuing to believe that nothing had undermined the commission’s essential finding that Oswald was, in Slawson’s words, a “true lone wolf” who had acted without the knowledge or encouragement of others—that there was no conspiracy.

Today, however, Slawson’s silence has ended once and for all. Half a century after the commission issued an 888-page final report that was supposed to convince the American people that the investigation had uncovered the truth about the president’s murder, Slawson has come to believe that the full truth is still not known. Now 83, he says he has been shocked by the recent, belated discovery of how much evidence was withheld from the commission—from him, specifically—by the CIA and other government agencies, and how that rewrites the history of the Kennedy assassination.

Slawson is now wrestling with questions he hoped he would never have to confront: Was the commission’s final report, in fundamental ways, wrong? And might the assassination threat have been thwarted? The commission, he believes, was the victim of a “massive cover-up” by government officials who wanted to hide the fact that, had they simply acted on the evidence in front of them in November 1963, the assassination might have been prevented. “It’s amazing—it’s terrible—to discover all of this 50 years late,” says Slawson, whose health is still good and whose memories of his work on the commission remain sharp.

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.

He has also come to believe—again, only recently—that the CIA knew about these meetings but hid the evidence of them from the Warren Commission.

What has changed Slawson’s mind so dramatically on questions that he thought were settled half a century ago? I interviewed him repeatedly, over several years, for my 2013 book on the Kennedy assassination, and Slawson says that our conversations, as well as material that I had gathered from declassified government archives and from other researchers, shook his confidence. “It never occurred to me until you interviewed me and I read your book that the commission’s investigation had been blocked like this.” It never occurred to him, he said, that the CIA and other agencies “tried to sabotage us like this.”

It was clear to me from the earliest days of my research on the book just how much I would want Slawson’s cooperation. It is hard to overstate his significance in the work on the commission—and in the investigation’s finding that Oswald acted alone. Although he had been the junior member of the two-lawyer team that focused on a possible foreign conspiracy, the work fell almost entirely to Slawson. His senior partner appeared in the commission’s offices only one day a week, according to the commission’s records, and Slawson finished up doing “90 percent of the work,” he told me.

In 2010, after two years of gathering up tens of thousands of once-classified documents from the National Archives and elsewhere, I made the first of several transcontinental reporting trips to meet with Slawson at his home in Washington State, where he moved after his retirement from USC. Each time, I brought with me the latest batch of documents that I had retrieved. And after each trip, Slawson grew more and more alarmed to discover how much evidence about the assassination—and specifically, about Oswald and the possibility of a conspiracy—had not been shared with him in 1964.

That year, the CIA told the commission that Oswald, a self-proclaimed Marxist who had apparently gone to Mexico to get visas that would allow him to defect to Cuba, had come under limited surveillance by the agency’s Mexico City station after he made appearances at both the Cuban and Soviet embassies there. But CIA documents declassified in the 1990s suggested that the agency had Oswald under far more aggressive surveillance in Mexico than it admitted to the commission. After reading these documents, Slawson now believes that the spy agency doctored evidence, including tapes of wiretapped phone calls in Mexico, that would have shown that the CIA knew before the assassination about the danger that Oswald posed.

He was outraged, in particular, when I showed him an eye-popping June 1964 letter from FBI Director J. Edgar Hoover to the commission that described how Oswald, in an outburst at a Cuban diplomatic compound in Mexico City during his trip there, had reportedly been overheard threatening, “I’m going to kill Kennedy.” According to the letter, a secret FBI informant had heard about the outburst directly from Fidel Castro during a meeting with the Cuban dictator in Havana several months after the JFK assassination. (The informant would later be revealed to be a leader of the American Communist Party.)

Slawson was certain he had never before seen the Hoover letter, even though it was written in the middle of the Warren Commission’s work. I explained to Slawson that I had found the two-page Hoover letter in the declassified files of the CIA—if it ever existed in the commission’s files, the physical copy had disappeared—and that I sensed instantly that it was a bombshell. I showed it to Slawson because I could not understand why he had not followed up on it in 1964.

“Obviously, somebody intercepted that letter before it could reach me,” Slawson told me. Even though the letter might not prove there was a conspiracy, Slawson said that if he had seen it, he would have raised “many, many questions” about who else knew that Oswald—a former Marine with rifle training, a champion of Castro’s revolution looking for a way to demonstrate his loyalty to Cuba—was apparently talking openly about killing the president. Slawson says he would have insisted that the FBI and CIA try to track down anyone in Mexico who might have known about the threat to Kennedy’s life. “I never had the chance to follow up because I never knew any of this,” he says.

Slawson feels betrayed by several senior government officials, especially at the CIA, whom he says he trusted in 1964 to tell the truth. He is most angry with one man—then-Attorney General Robert Kennedy, who assured the commission during the investigation that he knew of no evidence of a conspiracy in his brother’s death. It is now clear, as I and others have reported, that Robert Kennedy withheld vital information from the investigation: While he publicly supported the commission’s findings, Kennedy’s family and friends have confirmed in recent years that he was in fact harshly critical of the commission and believed that the investigation had missed evidence that might have pointed to a conspiracy.

“What a bastard,” Slawson says today of Robert Kennedy. “This is a man I once had admiration for.”

Slawson theorizes that that attorney general and the CIA worked together to hide information about Oswald’s Mexico trip from the commission because they feared that the investigation might stumble onto the fact that JFK’s administration had been trying, for years, sometimes with the help of the Mafia, to assassinate Castro. Mexico had been a staging area for the Castro plots. Public disclosure of the plots, Slawson says, could have derailed, if not destroyed, Robert Kennedy’s political career; he had led his brother’s secret war against Castro and, as declassified documents would later show, was well aware of the Mafia’s involvement in the CIA’s often harebrained schemes to murder the Cuban dictator. “You can’t distinguish between Bobby and the CIA on this,” Slawson says. “They were working hand in glove to hide information from us.”

The June 1964 letter from FBI Director J. Edgar Hoover about Oswald threatening to kill Kennedy.

Although there is nothing in the public record to show that Robert Kennedy had specific evidence of a foreign conspiracy in his brother’s death, I agree with Slawson that RFK and senior CIA officials threw the commission off the trail of witnesses and evidence that might have pointed to a conspiracy, especially in Mexico. Slawson also now suspects—but admits again that he cannot prove—that Chief Justice Earl Warren, who led the commission that bore his name, was an unwitting participant in the cover-up, agreeing with the CIA or RFK to make sure that the commission did not pursue certain evidence. Warren, he suspects, was given few details about why the commission’s investigation had to be limited. “He was probably just told that vital national interests” were at stake—that certain lines of investigation in Mexico had to be curtained because they might inadvertently reveal sensitive U.S. spy operations.

That might explain what Slawson saw as Warren’s most baffling decision during the investigation—his refusal to allow Slawson to interview a young Mexican woman who worked in the Cuban consulate in Mexico and who dealt face-to-face with Oswald on his visa application; declassified CIA records would later suggest that Oswald had a brief affair with the woman, who was herself a committed Socialist, and that she had introduced him to a network of other Castro supporters in Mexico. “It was a different time,” Slawson says. “We were more naïve. Warren would have believed what he was told.”

The theory that a conspiracy to kill JFK was hatched in Mexico is not new. Commission records show that during the course of the investigation, another former commission staffer, David Belin, who died in 1999, also suspected that Oswald had accomplices in Mexico and that they may have been waiting on the Mexican border after the assassination to help Oswald escape. Still, Slawson is the first surviving commission staffer to suggest the conspiracy in such a public fashion, and his credibility is obviously enhanced by the fact that he was the commission’s chief conspiracy-hunter.

Despite all that he has learned in recent years, Slawson is not hard on his “naïve” 33-year-old self. He says he still remains proud of his own contribution to the Warren Commission and its final report. “I know I did the best I could,” he says. “I had no way of knowing what I wasn’t being told.”

He says he has some confidence that the mistakes of 50 years ago would probably not be repeated now, if only because the American public is so much more cynical today about the government and its truthfulness. The national traumas and scandals that followed the Kennedy assassination—the Vietnam War, Watergate, the Iran-Contra affair, the 9/11 attacks terrorists and the huge intelligence failures that preceded both 9/11 and the disastrous American invasion of Iraq in 2003—have all seen to that.

In 1964, “we assumed that government officials would tell us the truth,” Slawson says. Half a century later, “no one makes that assumption anymore.”