"Film is truth at 24 frames per second," declared the New Wave director Jean-Luc Godard in 1963. The same year, a Dallas businessman named Abraham Zapruder entered American history on November 22 by standing in Dealey Plaza with his 8-millimeter camera as the motorcade of John F. Kennedy passed by. Mr. Zapruder shot 27 seconds of film that have burned themselves into public memory. After four decades, the images remain a source of inexhaustible fascination for some viewers, with each frame offering clues to what happened that morning -- or, perhaps, deepening the mystery.

"Not all murders get solved, especially after 40 years," says David R. Wrone, a professor emeritus of history at the University of Wisconsin at Stevens Point, whose new book, The Zapruder Film: Reframing JFK's Assassination, was published this month by the University Press of Kansas. He began having doubts about the government's handling of the case even before the Warren Commission issued its report concluding that Lee Harvey Oswald had acted alone in murdering the president. "The evidence shows that two or more individuals are responsible for his death," says Mr. Wrone. "The shots came from behind and in front. But the facts alone don't tell you who killed him. I don't know who it was, or why."

Mr. Wrone's reluctance to speculate on that score distinguishes his book from the abundant literature of conspiracy theory. But it otherwise pursues some familiar arguments from that genre. Members of the "assassination research community" (as they prefer to call themselves) have long disputed the Warren Commission's finding that frame 210 of the Zapruder film records the first shot at the motorcade. Mr. Wrone suggests that the shot occurred as early as frame 190. That is a difference of about one second, but he says it places the commission's tightly constructed chronology into question. Mr. Wrone also scrutinizes frame 313 -- the ghastly image of the president's head being blown open -- and concludes that the fatal shot must have been fired from the legendary vicinity known as the "grassy knoll."

Much of Mr. Wrone's book traces the provenance of the Zapruder film, which was confiscated by the FBI immediately after the assassination and seldom viewed by the public until the 1970s. He devotes a chapter to disputing the claims of a school of assassination theorists he calls the "alterationists," who contend that the Zapruder film was tampered with as part of a coverup. "People with basic common sense are appalled when they hear about it," says Mr. Wrone. "They conclude that all inquiries into Kennedy's death are akin to this sort of thing. It more or less poisons the well."

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One "alterationist" Mr. Wrone mentions is James H. Fetzer, a professor of philosophy at the University of Minnesota at Duluth. But to judge from Mr. Fetzer's recent work, the more accurate label might be "fabricationist." Serious assassination researchers now understand, says Mr. Fetzer, "that the autopsy X-rays have been fabricated, that there was a substitution of another brain for JFK's, that Oswald was framed using phony evidence, and that the Zapruder film has been fabricated." Mr. Fetzer organized a conference held at his campus in May, the proceedings of which have just appeared in The Great Zapruder Film Hoax: Deceit and Deception in the Death of JFK, the third of his books on the assassination, all published by Open Court.

A conversation with Mr. Fetzer quickly turns into a minutely detailed lecture on questions of ballistics, cinematic special effects, top-secret CIA film-development labs, and the trajectory of brain fragments flying out of a man's head -- all delivered at such high speed that a listener may experience hyperventilation by proxy. Abraham Zapruder "may or may not have taken a film," Mr. Fetzer says, "but the version we have now has been put together from various sources." While squads of marksmen had the president in their sites, so, by Mr. Fetzer's account, did teams of camera operators, all standing in the vicinity of Mr. Zapruder. The shots were later edited together to produce a document that has, it seems, fooled simpler viewers, but not the assassinologists who convened in Duluth earlier this year.

In addition to his voluminous work on the Kennedy assassination, Mr. Fetzer's curriculum vitae includes dozens of publications on the philosophy of science, with special emphasis on cognitive research and artificial intelligence. It does not appear, however, that he has devoted any scholarly attention to Occam's razor, also known as the principle of parsimonious explanation.

Possibly even wilder in his approach, but with a much keener sense of irony, is David M. Lubin, a professor of art and culture at Wake Forest University. In Shooting Kennedy: JFK and the Culture of Images, published this month by the University of California Press, Mr. Lubin treats the Zapruder film as just another piece of visual culture, caught up in the mosaic of images glowing in the Kennedy aura.

In a moment of willful perversity, Mr. Lubin notes "a strange but striking resemblance" between parts of the Zapruder film and "the title sequence of the weekly TV series that was then watched by more Americans than any other, The Beverly Hillbillies." The Kennedys and the Clampetts were, he writes, "originally poor American families that struck it rich. Nouveau riche, that is. Swimmin' pools! Movie stars!"

Mr. Lubin says he is "definitely fascinated by the whole conspiracy mystique." But his goal, he says "was never to figure out who really shot JR. Or, excuse me, JFK."