A former Secret Service agent who witnessed John F. Kennedy's assassination up close has shared shocking new details about one of the most mysterious events in American history.

In an upcoming memoir, former agent Paul Landis says he retrieved the so-called "magic bullet" from the back seat of Kennedy's vehicle.

The finding calls into question the so-called "magic bullet theory" or single bullet theory, which has long been seen as a critical piece of the conventional narrative that Lee Harvey Oswald acted alone.

The "magic bullet"

The theory posits that a single bullet caused Kennedy's initial neck wound and all of the injuries to Texas governor John Connally, who was seated in front of Kennedy in his limousine as it traveled through Dealey Plaza in Dallas.

The claim has been labeled the "magic bullet theory" by skeptics, referring to the alleged improbability of the bullet's path and the pristine condition in which it was found.

The Warren Commission, which endorsed the theory in its contested report on the assassination, based the theory on the fact that the bullet was found in Connally's gurney at the hospital.

Agent comes forward....

But Landis, who was guarding Jackie Kennedy that day, tells the New York Times that he found the bullet in the back seat of Kennedy's car and placed it on the president's stretcher at the hospital.

In other words, Kennedy and Connally may not have been injured by the same bullet.

Landis said he believes the "magic bullet" failed to penetrate Kennedy's back and then fell out. He said he left it with the president's body to preserve it for investigators.

“There was nobody there to secure the scene, and that was a big, big bother to me,” Landis said. “All the agents that were there were focused on the president.”

Did Oswald act alone?

Landis was never interviewed by the Warren Commission, and James Robenalt, a lawyer and author who helped prepare Landis' memoir The Final Witness, said the former agent was too traumatized in the aftermath of the assassination to follow up.

"He was totally sleep deprived and was still required to work, and was suffering from severe PTSD," Robenalt told the BBC. "He forgot about the bullet."

If the "magic bullet" did not cause both Kennedy's and Connally's injuries, skeptics say, there would not have been enough time for Oswald to fire both shots consecutively. That suggests there was one more than shooter.

Landis says he has always believed Oswald acted alone, but he's having second thoughts.

“At this point, I’m beginning to doubt myself,” he said. “Now I begin to wonder.”