A Secret Service agent on First Lady Jacqueline Kennedy’s security detail the day of President Kennedy’s murder, Paul Landis, is alleging multiple shooters pulled off the assassination. The claim is fodder for conspiracy theorists and a boon for his upcoming book, “The Final Witness,” but no smoking gun.

Mr. Landis told the New York Times, as reported in the Sun, that “the official account of Kennedy’s assassination does not align with what he says he experienced that day as he stood just feet from the slain president’s limousine.”

On November 22, 1963, Mr. Landis says he dug an intact bullet from Mrs. Kennedy’s seat in the car and carried it to Parkland Memorial Hospital. “There was nobody there to secure the scene,” he told the Times. “All the agents that were there were focused on the president.”

Mr. Landis, however, was among the agents tasked with that very job. He told the Times that he recognized the bullet was “important evidence” but that he didn’t preserve it. Instead, he wrapped it in a sheet on a gurney next to the president’s remains and “never mentioned” it to anyone.

The provenance of this so-called magic bullet went unexplained for decades, spawning speculation that it had been planted. Neutron activation tests in the late 1970s debunked this theory, matching it to Oswald’s rifle, which would eliminate it as proof of the second shooter Mr. Landis now proposes.

The Pulitzer Prize finalist Gerald Posner, author of  “Case Closed: Lee Harvey Oswald and the Assassination of JFK,” tells me he has “mixed feelings about what Landis has done,” noting that “in his book, he dramatically alters and adds to two detailed statements” he made days after the assassination.

The new allegations are “no match for the facts,” Mr. Posner says. They include errors such as the location where Kennedy was treated. Mr. Posner also cites testimony from medical staff that Kennedy’s “clothes were removed” before he was wrapped and put into a casket. “If Landis had put the bullet where he now claims, it would have ended up in the laundry hamper. … It could not have been JFK’s stretcher.”

In 2010’s “The Kennedy Detail,” Mr. Landis first debuted his sensational tale. He said then that he found only a “bullet fragment.” Citing pre-publication proofs of the 88-year-old’s book, Mr. Posner says that “with his memory somehow getting better with the passage of decades,” the former agent now “offers a more dramatic and detailed version.”

Mr. Posner sees “landmines” being created by Mr. Landis in the historical record, such as suggesting the bullet hit Kennedy but remained pristine. Expect, Mr. Posner says, the claim that assassin Lee Harvey Oswald removed gunpowder to weaken a single bullet to be “the launching pad for a new conspiracy theory.”

Of not securing the evidence, Mr. Landis writes that he thought the bullet “belonged” with the slain president and that leaving it on the gurney would assist in the autopsy. “Landis knew there would be no autopsy in Texas,” Mr. Posner says. The president’s detail made clear, over the objection of Dallas officials, that they would take his body back to Washington.

Four American presidents have been killed by assassins, but only the Kennedy slaying so captures the imagination. The simplest explanation which the preponderance of evidence supports — that another angry man changed history with the pull of a trigger — is just not romantic enough to fit the Camelot myth.

“The shame of all of this,” Mr. Posner said, “is that Landis did not own up to whatever he did at the time it happened. That was when it really mattered, when everyone else was alive and investigators could have tracked his story and sorted it all out. Now, we will never know with certainty.”

Expect the conspiracy peddlers to sow new seeds in that doubt, created by a Secret Service agent who says he concealed evidence for 60 years. It’s a gift to those who seek to exonerate Oswald but a disservice to the president Mr. Landis served, who deserves to have his assassin shamed in history and the case of his murder closed for good.