There are conspiracy theories galore surrounding the assassination of the 35th US president John Fitzgerald Kennedy, which shocked the world on this day 54 years ago.

They include the involvement of the CIA, vice-president Lyndon Johnson, the Cuban Prime Minister Fidel Castro or the KGB - but few have considered the possibility of JFK's wife Jackie being his killer.

More than half a century on, most independent investigators agree that 24-year-old former US marine Lee Harvey Oswald, the man accused of JFK's murder, was either an innocent 'patsy', or not the only person to shoot at Kennedy that day.

The Warren Commission was set up to investigate the assassination, but there were so many questions left unanswered that many still believe the commission was established not to uncover the facts but to hide them.

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Could the truth be that it was Jackie, hired by the CIA, who fired the fatal shot into Kennedy's throat that sunny autumn morning in 1963?

In the official report, it's said that Oswald - who was shot dead two days later by nightclub owner Jack Ruby - fired at the presidential limo twice, sniper-style, from the 6th floor of the Texas Book Depository, one shot missing the car and the other killing the president and injuring Governor John Connally, who was also in the open-top car as it cruised down Dealey Plaza on November 22nd 1963.

But, for some, the facts just don't add up. Dr. James Fetzer, author of Murder In Dealey Plaza, suggests the assassin was definitely not Oswald.

He says: "According to the Warren Report, Lee Oswald scored two hits, firing three shots in six seconds… with a medium to low velocity weapon, which couldn't have inflicted the damage to the President's skull."

Skilled master marksmen tried to replicate the scenario but failed. What the report claimed Oswald achieved was actually impossible, the MailOnline reported.

Is Dr. Fetzer suggesting that Kennedy was more likely to have been shot at close range, shot by the person sitting next to him in the car that day perhaps?

Although Fetzer's theory, while casting doubt on the official version, does not point the finger directly at Kennedy's wife, some believe it was Jackie who was behind the fatal gunshot.

The Warren Commission states that two shots were fired from behind, yet Kennedy's injuries it is claimed showed he was shot from the front, with the bullet making an exit at the back of his skull.

Fetzer says: "The autopsy x-rays were fabricated to conceal a massive blow-out to the back of the head, caused by a shot fired from in front… The autopsy record was falsified."

In fact, over 40 witnesses described the fatal injury as a gaping wound, the size of a baseball, at the back of the president's head, but the autopsy photos showed no such wound, just a small round hole at the back of his neck.

Journalist and best-selling author Dr. Jim Marrs, who died earlier this year, has written about the JFK conspiracy in his book Crossfire, which was used as a source for Oliver Stone's movie JFK.

He agrees with Dr. Fetzer that "Kennedy was shot from the front, not the back."

Despite these repeated claims, the media have consistently branded anyone questioning the official story as a crackpot.

To suggest that the much-loved First Lady was responsible for such a heinous act would be something even the most hardened conspiracy theorist would have trouble getting to grips with.

But the Warren Commission would not waver: it couldn't. If it agreed that JFK was shot from the front, it would have to admit that there was more than one assassin, one who shot the president from the front.

For anyone interested in the story, first stop for evidence would be the now-famous Zapruder film.

Dallas dress manufacturer Abraham Zapruder was said to be a bystander on Dealey Plaza, filming the motorcade as it passed by. The film, confiscated immediately after the assassination and not shown to the public until 1975, has now been seen by millions worldwide.

For years, the public believed that the film showed the events of those distressing few minutes as unedited, real-time evidence.

But some of those who've since studied the shocking footage closely have claimed that, not only has the film been edited, but that First Lady Jackie behaves in a most peculiar way.

In the film, the viewer can see the limo gliding along the plaza. As the motorcade approaches, John Kennedy seems to be in some kind of distress, as if he may be having trouble breathing.

Wife Jackie moves forward in her seat and turns 90 degrees to face her husband.

Conspiracy theorists then describe how she then leans in towards him, seemingly raising her left hand towards his throat. Suddenly – a flash of orange, the crack of a gunshot and Kennedy slumps forwards and to one side.

At that point, instead of ducking down and taking cover as one might in such a situation, Jackie kneels up on the seat and clambers over the back of the car, and the conspiracy theorists claim, something can seen flashing in her left hand.

A bodyguard – Clint Hill - leans forward, before Jackie does a clumsy crawl backwards into her seat.

The most popular narrative used to explain her movements is that, in a state of shock, she was frantically trying to retrieve some of her husband's brain and skull.

Most people seem happy to buy into that explanation. But is this what was really happening? Many conspiracy theorists think not.

There's one YouTube video which considers the idea that Jackie may have been the killer, but this film is a home-made documentary, narrated by a man with a scruffy beard and wearing a T-shirt, a look which causes many viewers not to take him as seriously as they would a man in a suit.

The video looks into a phenomena known as The Mandela Effect and from 0:53 in this video, he examines the Zapruder footage, freeze-framing it to show what he claims to be something shiny and possibly metallic in Jackie Kennedy's left hand.

But was Jackie left-handed? Well, as it happens, according to various sources on the internet, she was.

Another video claims that the Zapruder footage appears to have been doctored or even totally reconstructed and re-shot, with conspiracy theorists suggesting it uses actors in an attempt to cover up the real events of that day. It also touches on how Jackie may have been JFK's assassin.

But why would Jackie Kennedy want to kill her husband? Several reasons have been bandied about. It was no secret that 'Jack', as she called him, had affairs, the most notorious being with Hollywood legend Marilyn Monroe.

When Monroe was found dead – allegedly through suicide – Jackie didn't hide the fact that she was pleased and was reported as saying: "I was glad that Marilyn got out of her misery."

The CIA had plenty of reasons to want to dispose of the president. He'd made no bones about his plans to take away some of their powers, especially when it came to the planning and execution of covert operations.

He'd even said quite publicly that he intended to "smash the CIA into a thousand pieces and scatter it to the winds." Could the CIA have persuaded an already angry and jealous Jackie into killing her unfaithful husband?

The JFK files have now been released to the public, so people can look into what happened for themselves, but as there are almost 3,000 files, it could prove time-consuming. For most, who killed JFK – the CIA, the KGB, Oswald or Jackie - will remain a mystery.