REASONABLE DOUBT; An Investigation into the Assassination of John F. Kennedy. By Henry Hurt. Holt, Rinehart and Winston. 555 pp. $19.95.

BARELY A MONTH after John Kennedy's assassination, I arrived in Christchurch, New Zealand, on a reporting trip for The New York Times. Since breaking news was hard to come by in that fair, bucolic land, I was delighted to stumble on a small group of expatriate Americans, who had gone there to escape the threat of nuclear war in the Northern Hemisphere. One night at their bungalow on the city's edge, the conversation turned to the president's death. It was from them that I first encountered profound distrust of the official version. And their suspicion went far beyond the assassination itself to encompass the very agencies charged with investigating the crime, the integrity of the American system itself.

"So what are you really saying?" I finally asked in some exasperation. "That the CIA, the Texas oil industry and the South Vietnamese government joined hands to kill the president?" "Possible," said an intense young Midwesterner. "That's what you have to understand. Anything is possible."

I thought of that conversation the other day when I finished Reasonable Doubt, Henry Hurt's major new study of the Kennedy assassination. After 555 pages exhaustively analyzing the two-gunman theory, the "magic bullet," Officer Tippit's clipboard, David Ferrie's alibi, Jack Ruby's long distance phone calls, Clay Shaw's whips and chains, I could come to only one rational conclusion: anything is possible.

If one accepts Henry Hurt's analysis, the only scenario which is virtually impossible is the Warren Commission's conclusion: that Lee Harvey Oswald was solely responsible for killing Jack Kennedy.

Hurt takes pains to emphasize that he is not part of the grassy-knoll crew, that band of stalwart conspiracy theorists whom I first encountered in New Zealand. A roving editor for Readers Digest, he has written an earlier book about a Soviet defector and has a continuing interest in the intelligence community. But his only previous work on the Kennedy assassination was research for Edward Jay Epstein's Legend: The Secret World of Lee Harvey Oswald. Hurt concedes that, like many Americans, he had "a general feeling that the official version seemed illogically simplistic," but insists that during his early labors on Reasonable Doubt he "fully expected that at any moment I would encounter that single, unalterable piece of evidence that left no question that Oswald was the man who killed Kennedy."

Perhaps, but his suspension of judgment didn't seem to last very long. Throughout this volume, Hurt is openly contemptuous of the conventional version. He writes of the "gross incompetence" of the president's autopsy; the "whole corrupt package" of evidence in Officer Tippit's shooting; the "whole dismal debacle" of the destruction of military records; the "quicksands of deceit" into which the government kept slipping; the "familiar manipulation of facts to build a case against Oswald," and finally the Warren Commision's "flagrant disregard for the truth."

THERE IS ample reason, of course, to doubt the commission's received wisdom. Hurt is most convincing in his meticulous dissection of its scenario. Although little of his material is fresh, he skillfully marshals existing data to expose gaping lacunae in the argument. The bungled autopsy, for example, still astonishes after all these years. Cyril Wecht, a former president of the American Academy of Forensic Medicine, may not have been exaggerating much when he declared, "This is the kind of examination that would not be tolerated in a routine murder case by a good crew of homicide detectives in most major cities of America."

The ballistics studies seem just as slipshod. There remain good grounds to doubt Oswald's ability to perform such remarkably fast and accurate shooting with his old, clumsy Mannlicher-Carcano rifle, or for that matter with any weapon. Sherman Cooley, an expert hunter who served with Oswald in the Marine Corps, said, "If I had to pick one man in the whole United States to shoot me, I'd pick Oswald. I saw that man shoot, and there's no way he could have ever learned to shoot well enough to do what they accused him of."

But Hurt is less persuasive when he seeks to assemble an alternative scenario. Everyone in his story has a purpose, every event a larger meaning. There is little room for chance, for serendipity, for the random manner in which most people lead their lives.

And the only major piece of new evidence in Reasonable Doubt is singularly unconvincing. This is the testimony of one Robert Wilfred Easterling, who has told Hurt at great length of his role in a conspiracy to kill the president. According to Easterling, he was approached at the Habana Bar in New Orleans in Febrary 1963 by one Manuel Rivera, apparently an agent for the Cuban government, who promised to pay him well for his help in the assassination. It was Rivera himself who did the shooting, Easterling says, but the elaborate plan involved firing Oswald's rifle into a barrel of water to obtain bullets which could later be used to mislead investigators; an Oswald "clone" who made himself conspicuous at the Texas Book Depository both before and after the assassination; and a large wooden box used to smuggle the real death weapon out of the Depository some time later.

Years later, Rivera's younger brother, Francisco, encountered Easterling in San Pedro Sula, Honduras. After drinking for several hours at a hotel bar, Francisco told Easterling that he had seen his picture on the wall of Raul Castro's den. To prove this, he pulled from his pocket a small portfolio of photographs showing the items on Raul's wall -- pictures of Easterling, Oswald, Ruby, David Ferrie, Manuel Rivera and still another of a Czech-made rifle attached to a wooden board, above a mahogany plaque inscribed "Kennedy 1963."

By Hurt's own description, Easterling is "a terribly sullied witness." Parts of his story are "obviously preposterous." He is "a multiple felon, an ex-convict, a raging alcoholic, a diagnosed psychotic and schizophrenic," who has been committed to mental institutions on several occasions. Yet Hurt chastizes the FBI for failing to take Easterling's story seriously.

In the final analysis, Hurt concedes, "It is not possible to prove that Easterling's confession is true." Indeed, "it is useless, if not foolish, to attempt to argue conclusively in favor of a particular theory. . . . Too much pertinent evidence is either missing, destroyed, or languishing under seals of national security. Hope for a final answer must be held in abeyance until the day when there is full access to those secrets."

In the meantime, anything and everything is possible.