As one of two Secret Service agents assigned to safeguard the wife of President John F. Kennedy, Paul Landis was never far behind Jackie Kennedy wherever she went.

That was the situation on November 22, 1963, when the Kennedys embarked on what should have been a routine motorcade through the streets of Dallas, Texas.

As excited crowds clamoured to catch a glimpse of America's golden couple in their open-top limousine, 28-year-old Landis was one of four agents keeping watch from the jumpboard of a Cadillac immediately behind.

It meant that when shots rang out, he had a ringside view of one of the most shocking — and defining — events of the 20th century.

Yet for almost 60 years Landis has remained largely silent, traumatised by what he witnessed after the limousine passed by the Texas School Book Depository and bullets struck the President's neck and head. It is only now, in his 88th year, that he feels able to fully recall the day President Kennedy was assassinated in front of him.

This month, Landis publishes his book, The Final Witness, a compelling account of his time in the Kennedy detail, the elite team whose mission was to protect the President and the First Lady.

Why did it take him so long to tell his story?

'I had nightmares for years about the President's head exploding in front of me, so I tried to remove myself from the whole situation,' Landis told the Mail, speaking exclusively from his home in Cleveland, Ohio.

Intriguingly, however, his memories throw doubt on the official Warren Commission account of what happened at 12.30pm that sunny autumn day.

In particular, Landis's recollections are at odds with the Commission's finding that one bullet — the so-called 'magic' bullet — was able to pass through John F. Kennedy then hit Texas Governor John Connally in multiple places before emerging undamaged.