There was a sudden, sharp, shattering sound. Various individuals heard it differently. Jacqueline Kennedy believed it was a motorcycle noise. Curry was under the impression that someone had fired a railroad torpedo. Ronald Fischer and Bob Edwards, assuming that it was a backfire, chuckled. Most of the hunters in the motorcade - Sorrels, Connally, Yarborough, Gonzalez, Albert Thomas - instinctively identified it as rifle fire.

But the White House Detail was confused. Their experience in outdoor shooting was limited to two qualification courses a year on a range in Washington's National Arboretum. There they heard only their own weapons, and they were unaccustomed to the bizarre effects that are created when small-arms fire echoes among unfamiliar structures - in this case, the buildings of Dealey Plaza. Emory Roberts recognized Oswald's first shot as a shot. So did Youngblood, whose alert response may have saved Lyndon Johnson's life. They were exceptions. The men in Halfback were bewildered. They glanced around uncertainly. Lawson, Kellerman, Greer, Ready, and Hill all thought that a firecracker had been exploded. The fact that this was a common reaction is no mitigation. It was the responsibility of James J. Rowley, Chief of the Secret Service, and Jerry Behn, Head of the White House Detail, to see that their agents were trained to cope with precisely this sort of emergency. They were supposed to be picked men, honed to a matchless edge. It was comprehensible that Roy Truly should dismiss the first shot as a cherry bomb. It was even fathomable that Patrolman James M. Chaney, mounted on a motorcycle six feet from the Lincoln, should think that another machine had backfired. Chaney was an ordinary policeman, not a Presidential bodyguard. The protection of the Chief Executive, on the other hand, was the profession of Secret Service agents. They existed for no other reason. Apart from Clint Hill - and perhaps Jack Ready, who started to step off the right running board and was ordered back by Roberts - the behaviour of the men in the follow-up car was unresponsive. Even more tragic was the perplexity of Roy Kellerman, the ranking agent in Dallas, and Bill Greer, who was under Kellerman's supervision. Kellerman and Greer were in a position to take swift evasive action, and for five terrible seconds they were immobilized.

Hill, though mistaken about the noise, saw Kennedy lurch forward and grab his neck. That was enough for Clint. With his extraordinary reflexes he leaped into Elm Street and charged forward.

Manchester and JFK became friends in 1946 while both were recovering from debilitating war wounds. During the 1950s and the "Camelot'' years, Manchester was a confidant and companion to Kennedy, and a frequent visitor to the family's compound in Hyannisport, Mass.

The friendship helped provide Manchester with material for his breakthrough book - the 1962 ``Portrait of a President,'' the first of three he wrote about the late president. The shattering experience of the Kennedy assassination the following year and an exhaustive, controversial investigation led to ``The Death of a President,'' published in 1967.

Jacqueline Kennedy made an unsuccessful attempt to block the publication, saying it revealed intimate family details. Manchester eventually agreed to drop certain passages. Still, the book sold more than a million copies.

Explaining the Kennedy mystique in "The Death of a President,'' Manchester wrote: "The nub of the matter was that Kennedy had met the emotional needs of his people. His achievements had been genuine. His dreams and his oratory had electrified a country grown stale and listless and a world drifting helplessly toward Armageddon.''

In a 1999 New York Times interview, he said he thought so many people believed Kennedy was killed in a conspiracy because of ``that dreadful Oliver Stone movie'' (``JFK'') and because people felt someone as insignificant as Lee Harvey Oswald couldn't have done such a momentous thing.

``If you put the murder of the president of the United States at one end of the scale, and you put that waif Oswald on the other end, it just doesn't balance,'' he said. ``And you want to put something on Oswald's side to make it balance. A conspiracy would do that beautifully. Unfortunately, there is no evidence whatever of that.''

In 1964 Mrs. Kennedy commissioned Mr. Manchester to produce an account of the assassination. She was familiar with Mr. Manchester's work mostly through his book "Portrait of a President: John F. Kennedy in Profile." Published two years earlier, it was an account of the president's first year and a half in the White House, one that many reviewers found to be adoring. Mr. Manchester had met and grown to admire Kennedy when both were recovering from war wounds in Boston.

Mrs. Kennedy promised him exclusive interviews with members of the family. The book agreement stipulated that his manuscript would be reviewed by Mrs. Kennedy and by the president's brother, Robert F. Kennedy, then attorney general and soon to become a United States senator from New York. As part of his agreement, Mr. Manchester would receive an advance of $36,000 but only against the income from the first printing. All other earnings would go the Kennedy Memorial Library.

"The Death of a President" was completed in 1966, and Mr. Manchester turned his manuscript over to his publisher, Harper & Row, and to the Kennedy family for review.

In the interim Mr. Manchester received an offer of more than $650,000 from Look magazine for first serial rights; his agent had obtained an agreement that payments for a serial would go to the author.

But Mrs. Kennedy balked at the serialization plans, saying that they smacked of rank commercialization, that she had not given her final approval and that she would seek a court injunction to block publication of the book.

Mrs. Kennedy's decision was a bombshell in the publishing world, and for weeks newspapers were filled with articles about her decision and speculation about the contents of "Death of a President," which had been eagerly awaited. Mrs. Kennedy did not say it publicly, but it was widely believed at the time that she feared that some passages in the book unsympathetic to Johnson might increase political tensions between him and Robert Kennedy, endangering Robert Kennedy's political aspirations.

In the weeks that followed, the Kennedy family resolved whatever problems it had with Mr. Manchester's book. Some deletions were made, trims that Mr. Manchester said were minimal.

Harper & Row published "Death of a President" in the spring of 1967. It became a best seller and later was given the Dag Hammarskjold International Literary Prize. It has sold more than 1.3 million copies in hardcover.

William Manchester, 82, whose riveting books about men in military and political life made him one of the greatest popular historians of the 20th century, died June 1 at his home in Middletown, Conn.

His slow death, after two strokes, brought a poignant end to one of the most productive and scrupulous writers of best-selling tomes about outsized modern historical figures and contemporary culture.

Fueled by yogurt and brief naps in his office, the sinewy Mr. Manchester could withstand 50-hour writing sessions in his heyday. In recent years, he was grief-stricken by his inability to concentrate even on simple television programs, much less his final, three-volume project, a biography of Winston Churchill. He had to relinquish control of his career-capping work.

"Language for me came as easily as breathing for 50 years, and I can't do it anymore," he told the New York Times in 2001. "The feeling is indescribable."