New book by a secret service agent guarding JACKIE KENNEDY describes her terror Nov. 22, 1963 when JFK's head was blown off right in front of her!  

In former Secret Service agent Clint Hill's new book, 'Mrs. Kennedy and Me: An Intimate Memoir,' Hill reveals gruesome first hand details of the day Kennedy died.

Hill recalls in the book that suddenly, he saw the President grab for his throat. "Somebody had fired a shot at the President, and I had to get myself between the shooter and the President and Mrs. Kennedy. Nothing else mattered."

Freaked out Jackie jumped onto the back of the limousine in a desperate attempt to escape death that spattered all around her.

 "Her eyes were filled with terror…She was reaching for something.

"She was reaching for a piece of the President's head."

Hill was assigned to protect Jackie, walking alongside the ill-fated car in Dallas when death rang out – as JFK was shot first in the neck, then in the head, exploding his skull.

When Hill reached Jackie, he wrote, "She said, 'They shot his head off!'  'Oh, Jack, what have they done?'"

After taking the hysterical First Lady in his arms, Hill wrote that his initial thought was, "How did I let this happen to her?"