The KENNEDY clan is terrified over an upcoming book that promises to name the killer of JFK’s White House mistress MARY PINCHOT MEYER!

Massachusetts author Peter Janney claims to have solved the mystery behind the murder of the 43-year-old socialite, who carried on a two-year affair with President John F. Kennedy while he was in office.

While the Kennedy family is not implicated in the crime, they do not want to revisit sordid details of the past that will be revealed in the book, noted the source.

“It’s sure to open old wounds,” the source said about “Mary’s Mosaic: Mary Pinchot Meyer & John F. Kennedy and their Vision for World Peace.”

“The book will describe how JFK cheated on first lady Jackie – and then all the murky details of how gorgeous blonde Mary was killed almost a year after his assassina­tion.”

Incredibly, The ENQUIRER was the FIRST to break the news of the steamy relationship in our March 2, 1976, issue, and our scoop inspired author Janney.

“I have been researching her story since The ENQUIRER story first broke in 1976,” Jan­ney admitted.

The affair in­cluded almost 30 “trysts,” and it’s been reported that Mary brought marijuana or the mind-altering drug LSD for the sexual encounters.

On Oct. 12, 1964, 11 months after the Kennedy assas­sination, and two weeks after the Warren Commission report on his death was made public, Mary was shot to death while out for a walk in Washington.

A man named Raymond Crump was arrested, but the gun used in the shooting was never located and Crump was acquitted of all charges. The murder has remained unsolved ever since.

“Janney’s book will finally uncover the CIA’s suspected role in Mary’s death and will drop other bombshells,” said the source. “The Kennedys are bracing for the disclosures.”