Exactly fifty years ago Clint Hill was the Secret Service agent assigned to protect Jackie Kennedy.
Their destination was Dallas.
As the Presidential motorcade swept into the city that fateful Friday morning, Hill was positioned in a follow-up car, just behind the limousine.
The sound of the first shot startled Clint but he instantly raced towards JFK's car, jumping on the back to try and smother the President and the First Lady, to protect them from more gunfire.
Despite his quick reactions, he was one - or possibly two - seconds too late.
The third bullet from the sniper hit JFK in the head, covering Clint in blood and bone fragments.
He knew instantly that President Kennedy was dying.
He heard the First Lady gasp and say, "Jack, what have they done to you?"
Watch Clint Hill talk about the moments before, during and after the shooting:
For Clint those catastrophic seconds - an event so shocking it will never be forgotten by anyone of that generation - were the start of a lifelong nightmare.
Could he have moved faster?
Could he have taken the third bullet and saved the President?
These are questions that still haunt Clint Hill to this day.
He was speaking to me in Detroit, where the JFK Presidential limousine is still kept.
It's been modified since 1963, with a new glass roof and additional armoured panels.
But for Clint it still brings back powerful memories of that tragic November day in Dallas.
The retired agent says he now understands that he did his duty in Texas and that there was nothing he could have done differently.
There is no longer the gnawing guilt.
But the deep regret lingers on.
It is still a terrible burden being an eyewitness to one of the most shocking events of the 20th Century.
The memories, he says, haunt him every day.
Clint knows they will remain with him until he dies.
Watch Clint Hill talk about the guilt he says will always haunt him about that day: