As the most notorious boss of the Chicago Mafia since Al Capone, Sam Giancana controlled a criminal empire stretching across America.

He had an army of thugs to call on — but for two days in November 1963, he asked for help from his little brother, Pepe.

He was only a humble, illegal bookie in the Giancana empire, but Sam needed someone to chauffeur him around while the mobsters who usually did so were out of town.

As Pepe later confided to family, those men had gone to Dallas to do a job for Giancana — one that would become a seminal moment of the 20th century.

From conversations overheard at his brother's mansion in Oak Park outside Chicago, Pepe worked out that their task was to help Lee Harvey Oswald assassinate John F Kennedy, then ensure that Oswald did not live to talk.

Since then, millions of words have been written about the killing of JFK: claims that the Mafia wanted revenge for his war on organised crime have vied with counter-theories that the president was killed by the CIA, the Pentagon, Fidel Castro, the Soviets and even Vice President Lyndon Johnson.

Now, at long last, we may be coming close to the truth. Filmmaker Nicholas Celozzi — the great-nephew of Sam Giancana — says his Great Uncle Pepe and other members of his family have finally confided to him exactly how the Mafia killed JFK — and just as importantly — allowed him to reveal it.

He is working with the Pulitzer Prize-winning screenwriter and director David Mamet to produce a new — and what they believe could be definitive — version of the final 48 hours of President Kennedy's life.