The assassination of US president John F Kennedy, one of the defining events of the 20th century, is about to return to the international spotlight. President Donald Trump is currently being petitioned by the family of diplomat Charles Thomas, who took his own life in 1971, for the release of declassified documents – due tomorrow – that could cast new light on the notorious killing of the golden boy of American politics.

But how are the deaths of these two men connected and what might we learn from the publication of the files?

How did John F Kennedy die?

Kennedy was killed by a sniper’s bullets while touring Dallas, Texas, in an open-top limousine on 22 November 1963. The president was shot from long-distance – once in the neck and once in the back of the head – to the horror of his wife Jackie sat beside him in their Lincoln Continental, his security detail and the crowd watching the motorcade’s progress through Dealey Plaza.

Lee Harvey Oswald, a former US marine and Marxist, was arrested for the killing before he in turn was murdered by local nightclub proprietor Jack Ruby two days later, during a live television broadcast of his transfer from Dallas County Jail.

In the immediate aftermath, President Lyndon Johnson established the Warren Commission to examine the murder. Chief Justice Earl Warren’s team investigated for 10 months before producing an 888-page report concluding that both Oswald and Ruby had acted alone.

What has happened since?

Kennedy’s murder has inspired a slew of conspiracy theories ever since: from the “single bullet theory” to the possibility of a second gunman lurking behind a nearby grassy knoll, to the possibility that the hit was ordered and then covered up by the CIA., or finally that it was sponsored by America’s Cold War rivals.

So many abounded that Congress passed the JFK Assassination Records Collection Act in 1992, requiring that all intelligence relating to the attack should be made public unless it represented a national security threat.

Millions of pages of investigative reports into the tragedy were published by the National Archives at the time, while thousands more, previously suppressed over security concerns, were due for release last October to mark the act’s 25th anniversary.

But Mr Trump postponed the publication for a further six months, giving the FBI and CIA until 26 April 2018 to assess any risk they might still pose.

Where does Charles Thomas enter the story?

Charles Thomas had been a promising US civil servant, known as “the diplomat from central casting” for his square-jawed good looks and perfect family.

After an 18-year career in which he served US embassies in Latin America and Africa, Thomas had high hopes of being made an ambassador.

Instead, he shot himself in the head in the bathroom of his family home in Washington on 12 April 1971, a complete shock to all who knew him.

He was thought to have been privately ashamed of being “selected out” of the State Department after missing out on a promotion, a reassignment he considered a humiliating demotion.

Tragically, the decision to remove Thomas from his role later appeared to have been ordered by mistake, a clerical error for which his widow Cynthia subsequently received a formal apology from President Gerald Ford.

How does Thomas relate to JFK?

The Thomas family is pressing Mr Trump for the new documents’ release in the hope that they might reveal, among other things, the truth about what drove the patriarch to suicide. The belief that they might stems from the fact that Charles Thomas campaigned energetically for the reopening of the investigation into the Kennedy assassination.

The assassination of JFK – in pictures Show all 8 1 /8 The assassination of JFK – in pictures The assassination of JFK – in pictures President John F. Kennedy and his wife Jacqueline Kennedy ride with secret agents in an open car motorcade shortly before the assassination, 22 November 1963 Getty The assassination of JFK – in pictures President John F. Kennedy and his wife Jacqueline Kennedy prior to his assassination Keystone/Getty The assassination of JFK – in pictures Kennedy is struck by an assassin's bullet as he travels through Dallas in a motorcade In the car next to him is his wife Jacqueline and in the front seat is Texas governor John Connally Three Lions/Hulton Archive/Getty The assassination of JFK – in pictures The view from the sixth floor window of the Texas School Book Depository in Dallas, from which Lee Harvey Oswald is thought to have assassinated Kennedy. This photograph was taken approximately one hour after the assassination Hulton Archive/Getty The assassination of JFK – in pictures Lee Harvey Oswald during a press conference after his arrest in Dallas. Lee Harvey Oswald was killed by Jack Ruby on 24 November on the eve of Kennedy's burial AFP/Getty The assassination of JFK – in pictures Lyndon B. Johnson takes the oath of office as President of the United States on the day of Kennedy's assassination. Jackie Kennedy is stood next to Johnson Getty The assassination of JFK – in pictures Kennedy's funeral procession goes into Arlington Cemetery in Washington Newsmakers/Getty The assassination of JFK – in pictures Kennedy's casket sat in the East Room of the White House Newsmakers/Getty

In the mid-1960s, Thomas had served in Mexico and there encountered evidence that Lee Harvey Oswald had met with Cuban and Soviet officials and known KGB spies when he crossed the border in September 1963, just weeks before the president’s death.

Thomas’s discovery, which he raised with superiors in 1964, threatened to undermine the Warren Commission’s assertion that Oswald had acted alone and opened up the possibility that communist forces might have assisted Oswald in his plot.

JFK’s alleged assailant had previously attempted to defect to both Cuba and the Soviet Union and had talked openly of his plan to kill Kennedy.

What might the documents reveal?

The Thomas family suspects that the filing “error” that saw Charles fired might have been a deliberate bid to silence him. “Washington overpowered my father like a steam shovel, tossing him into a heap like discarded dirt,” Zelda Thomas-Curti – the deceased’s youngest daughter, now a business consultant in Minneapolis – wrote in her letter to Mr Trump, appealing for the release of all JFK-related files by the National Archives.