Crucial JFK file which wasn't released to public and the secrets it contains

A trove of thousands of documents about the assassination of JFK have been released - apart from a particularly crucial one.

The Trump administration has finally lifted the lid on the events of that fateful day in Texas on November 22, 1963.

The 35th President of the United States, John F Kennedy, was gunned down in broad daylight as he sat in the rear of the open-top presidential limousine as it cruised through the Dallas streets.

Conspiracy theories have since swirled around the assassination for more than six decades, as while US Marine Lee Harvey Oswald was believed to have administered the fatal gunshot to the head, investigators believed there could have been another gunman involved.

The president was fatally wounded by a shot to the head (Getty Images)

Yet Oswald was killed two days after the assassination, meaning he couldn't exactly give the authorities any insight on his motivations, and theorists turned to point the blame at the CIA, the Mafia and political rivalries, amounting to some 42 groups, 82 assassins and 214 people being accused of being involved in some way.

President Trump subsequently signed an executive order almost immediately after coming back into office in January to release tens of thousands of the remaining unredacted documents relating to not only the assassination of Kennedy but his brother, presidential candidate Sen. Robert F. Kennedy, and civil rights leader, Martin Luther King Jr.

On Tuesday (March 18), the treasure trove of some 2,200 records were released, though historians said they need 'time' to comb through them all.

There's still one potentially crucial file missing (Getty Images)

Included in the files are typewritten records, handwritten notes and some bombshells like Oswald being described as a 'poor shot', while the Secret Service was warned Kennedy would be killed three months prior, and details of a top CIA agent who claimed the deep state was responsible.

Yet some say the information isn't entirely new and is missing one crucial piece of the puzzle.

James Johnston, author of Murder, Inc.: The CIA under John F. Kennedy told USA Today that there remains at least one glaring omission.

Johnston said he is aware of one document that exists, but still isn't in the public domain of the National Archives.

Donald Trump signed an executive order to declassify the files (ROBERTO SCHMIDT/AFP via Getty Images)

This relates the first one-on-one conversation between President Lyndon Johnson and CIA Director John McCone after Johnson came to office after Kennedy's death.

Johnston, who worked on the congressional Church Committee that investigated the CIA in 1975, told the news outlet that the CIA had promised to turn everything over to the National Archives in 1988.

"If it was going to embarrass the agency or tell a different story, they wouldn't have turned them over to the National Archives in the first place. And if they were withholding them before, I’m guessing they would continue to withhold them," he said.

But McCone was long suspected to have withheld information from the Warren Commission that was set up by President Johnson to investigate the murder - and kept his job as CIA director.

The commission came to the verdict that Oswald was not part of any conspiracy or terrorist group and was simply a former Marine who acted alone on a Marxist vendetta.

The missing document could help to answer lingering questions about whether Cuba was involved in the slaying, since JFK had tried to use the CIA to kill communist dictator Fidel Castro.