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.

What if the answers to the many, persistent questions surrounding the assassination of President John F. Kennedy lie not in Dallas or Washington, D.C., but in the streets of a foreign capital that most Americans have never associated with the president’s murder? Mexico City.

Only hours after shots rang out in Dealey Plaza on November 22, 1963, U.S. Ambassador Thomas C. Mann told colleagues in the American embassy in Mexico that he was certain Lee Harvey Oswald had not acted alone in killing JFK.


Oswald had visited Mexico City several weeks earlier, apparently to obtain a visa that would allow the self-proclaimed Marxist to defect to Cuba, and Mann, a veteran diplomat, suspected that a plot to kill Kennedy had been hatched on Mexican soil, during Oswald’s encounters there with Cuban diplomats and Mexicans who supported Fidel Castro’s revolution. How did Mann know about those meetings? It turned out the CIA had Oswald under surveillance in the Mexican capital after he had showed up at both the Cuban and Soviet embassies there.

Back at the State Department, however, a baffled Mann hit a brick wall. No one in Washington seemed interested in his suspicions, he would later complain to colleagues. And within days of the assassination, the ambassador received an astonishing top-secret message directly from Secretary of State Dean Rusk. According to Mann’s testimony years later to congressional investigators, Rusk ordered the embassy to shut down any investigation in Mexico that might “confirm or refute rumors of Cuban involvement in the assassination.” No reason was given for the order, the ambassador said.

Mann told the congressional investigators that he was under the impression that the same “incredible” shut-down order had been given by the CIA to the spy agency’s station chief in Mexico, Winston Scott. In memoirs quietly declassified in the 1990s, after his death, Scott confirmed that he, too, suspected that Oswald was an “agent” of a foreign power who may have been part of a conspiracy to kill Kennedy (though Scott did not suggest that the CIA’s investigation was shut down).

What happened in Mexico City in the weeks before JFK’s murder? It is clear from government files declassified in recent decades that Oswald’s six-day trip to Mexico was never adequately investigated by the CIA, the FBI and the State Department—and, as a result, by the Warren Commission, the panel named by President Lyndon Johnson to investigate the assassination. The question has been raised anew in recent weeks by a surprising source—the Warren Commission’s chief conspiracy hunter. And in fact, lots of evidence has accumulated over the years to suggest that historians, journalists and JFK buffs who are still trying to piece together clues about the president’s murder—whether from the memories of still-living witnesses or in the new tranche of assassination-related documents the National Archives is set to release in two years—would be wise to look to Mexico City.

In the half-century since the commission named for Chief Justice Earl Warren concluded that Oswald was the sole gunman in Dallas and that there was no evidence of a conspiracy, foreign or domestic, it is startling to discover how many credible government officials—beginning with Ambassador Mann and CIA station chief Scott—have suggested that evidence was missed in Mexico that could rewrite the history of the assassination. The list includes the late former FBI Director Clarence Kelley and former FBI Assistant Director William Sullivan, as well as David Belin, a former staff lawyer on the Warren Commission.

Last month, another commission staffer joined their ranks: David Slawson, a retired University of Southern California law professor who, 51 years ago, was the commission’s chief investigator searching for evidence that might have pointed to a foreign conspiracy in JFK’s murder. In interviews for a new edition of my 2013 history of the assassination, Slawson said he is now convinced the commission was the victim of a “massive cover-up” by the CIA and other agencies to hide evidence that might have identified people in Mexico City who knew and encouraged Oswald to carry out his threat when he returned to the United States.

Declassified government records back up Slawson’s suspicion of how much information was withheld in 1964, when senior officials at both the CIA and the FBI assured the commission that there was no evidence in Mexico—or anywhere else—to suggest that Oswald was anything other than a delusional lone wolf. In sworn testimony to the commission, FBI Director J. Edgar Hoover insisted that “there was nothing up to the time of the assassination that gave any indication that this man was a dangerous character who might do harm to the president.”

The records declassified decades later tell a very different story, and show just how much evidence about Oswald’s Mexico trip —including CIA tape recordings of wiretaps of Oswald’s phone calls in Mexico—never reached the commission. Although the spy agency assured the commission in 1964 that there were no surveillance photos of Oswald in Mexico, CIA Station Chief Scott, in his memoirs, strongly suggested that there were photos, and other CIA officials later told congressional investigators in the 1970s that they recalled seeing the pictures. CIA and FBI records, meanwhile, show that the agencies never tried to track down or interview key witnesses who had encountered Oswald in Mexico.

Slawson is also convinced that someone blocked him from seeing a top-secret June 1964 letter from Hoover to the commission in which Hoover revealed that Oswald may have openly boasted about his plans—“I’m going to kill Kennedy”—while in Mexico, apparently at the Cuban embassy. Slawson believes the CIA was desperate to shut down any investigation in Mexico City out of fear the Warren Commission might stumble onto evidence of the spy agency’s long-running schemes to murder Fidel Castro. (Mexico City had been a staging area for some of the plots.)

Slawson is careful to note that he is not suggesting any sort of far-flung, carefully laid-out conspiracy. For one thing, he notes, Oswald did not get the job he held at the time of the assassination, at the Texas School Book Depository, which was on the president’s motorcade route, until after he had returned to Texas from Mexico in early October 1963; the route itself was not announced until days before JFK’s arrival in Dallas.

Still, if Oswald openly boasted about his plans to kill JFK among people in Mexico, it would undermine the official story that he was a lone wolf whose plans to kill the president could never have been detected by the CIA or FBI. In Slawson’s mind, it could even raise the question of whether people in Mexico might have been charged as accessories in the murder if they had known about Oswald’s threats but did nothing to stop him.

Ambassador Mann appears to have had similar suspicions. After retiring from the State Department, he told House investigators in 1977 that he had never stopped believing that Oswald had been part of a conspiracy somehow linked to Cuba, and that the CIA and other agencies had refused to investigate Oswald’s activities in Mexico “because it would have resulted in the discovery of covert U.S. government action” that somehow involved Cuba.

In memoirs published in 1987, former FBI Director Clarence Kelley, Hoover’s immediate successor, revealed that, after having a chance to read through the bureau’s raw files on the Kennedy assassination, he, too, came to believe that Mexico held the key to unanswered questions about the president’s murder. “Oswald’s stay in Mexico City apparently shaped the man’s thinking irrevocably,” Kelley wrote.

He said he became convinced from the files that, during meetings with Cuban diplomats in Mexico, “Oswald definitely offered to kill President Kennedy,” and that he had probably made a similar offer during the same trip at a meeting at the Soviet embassy in Mexico. That did not mean that either communist government was behind the assassination, Kelley insisted. But it did mean that people in both the Cuban and Soviet embassies were aware, weeks before the assassination, that a young American—a former Marine with rifle training who was eager to be known as a champion of Castro’s revolution—was talking openly about killing the president.

Another top FBI official, former Assistant Director William Sullivan, who directed the bureau’s investigation of JFK’s murder, wrote in his own memoirs that “there were huge gaps” in the FBI’s investigation and that many of them involved Oswald’s trip south of the border. “We never found out what went on between Oswald and the Cubans in Mexico City,” Sullivan admitted.

After finishing his work on the commission, staff lawyer David Belin, who died in 1999, wrote in a little-publicized book that he came to believe that Oswald may have planned to head from Dallas back to Mexico by bus after the assassination because he had some promise of help from co-conspirators who were waiting on the Texas-Mexico border.

Belin’s theory, which he developed during his work on the commission, stemmed from his analysis of local bus schedules and of a bus transfer issued on the day of the assassination that was found in Oswald’s clothing. According to Belin, Oswald may have met with Cuban diplomats and others in Mexico City who saw the Kennedy administration as a mortal threat and who “promised financial and other support to Oswald if he was ever able to succeed” in killing the president. Belin said that, to his disappointment, there was no mention of his theory in the commission’s final report because, as he admitted, it was “pure speculation” that undermined Chief Justice Warren’s hopes to snuff out conspiracy allegations.

Belin’s theory would have made sense to another American official—diplomat Charles William Thomas, whose once-promising career was mysteriously derailed after he pressed colleagues in the U.S. embassy in Mexico to pursue unanswered questions about Oswald’s Mexico City trip. In late 1965, Thomas was told by a friend—a prominent Mexican writer, Elena Garro de Paz—that she had seen Oswald at a dance party during his visit to Mexico that was also attended by a Cuban diplomat who had spoken openly about his hope that someone would assassinate Kennedy. Thomas said he was also told that Oswald had a brief affair with a vivacious young Mexican woman, a committed Socialist, who worked in the Cuban consulate and who had introduced Oswald around town to other Castro supporters.

State Department and CIA records declassified in recent years show that the agencies rebuffed Thomas in his requests for a new investigation, which he continued to raise even after he left Mexico in 1967 for a new posting in Washington. Thomas was dismissed from the department in 1969, a decision that the State Department years later would acknowledge was made in error—the result of what the department insisted was a clerical mistake related to the misfiling of some of Thomas’s personnel records. The department’s admission would come only after Thomas, who struggled to establish a new career, committed suicide in 1971.

Congressional investigators, who later reviewed the case and obtained pension benefits for Thomas’s family, said they suspected, but could not prove, that the diplomat had actually been forced out because of his persistent, unwelcome effort to open a new investigation of Oswald’s activities in Mexico. “It was impossible to prove, though,” one of the congressional investigators told me when I was writing my book. “If he was forced out because of Mexico City, it was all done with a wink and a nod.”

Decades after Thomas’s death, the State Department would declassify internal memos that he had written to superiors, in which he had pleaded for someone to go back and reinvestigate Oswald’s Mexico trip. What Thomas had learned in Mexico would not, by itself, “prove that there was a conspiracy to assassinate President Kennedy,” he wrote. But he warned of what might happen if long-secret evidence suggesting a Mexican-born conspiracy in JFK’s murder ever became public. “Those who have tried to discredit the Warren Report could have a field day,” he wrote.

In 1975, Thomas’s widow received a formal apology from the White House. “The circumstances surrounding your husband’s death are a source of deepest regret to the government he served so loyally and so well,” the letter said. “I can only hope that the measures which came about as a result of this tragedy will prevent reoccurrences of this kind in the future.” The letter was signed by President Gerald R. Ford, who, as a rising Republican congressman from Michigan in 1964, had been a member of the Warren Commission.

Thomas’s family may have reason to hope for even greater justice for the late diplomat, since so many of the people who encountered Oswald during his mysterious trip to Mexico half a century ago were young at the time and are still alive. I found some of them for my book, including people who suggest that Oswald had many more contacts with people in Mexico City who might have wanted to see JFK dead. The National Archives faces a 2017 deadline to release about 1,200 documents related to the assassination, many of them from the CIA, that still remain classified. While refusing to describe what is in the documents, CIA lawyers have acknowledged over the years that many of them are out of the files of agency employees who were stationed in the early 1960s in, of all places, Mexico City.