The book is scheduled to be published on Monday, and Mr. Marable had been looking forward to leading a vigorous public discussion of his ideas. But on Friday Mr. Marable, 60, died in a hospital in New York as a result of medical problems he thought he had overcome. Officials at Viking, which is publishing the book, said he was able to look at it before he died. But as his health wavered, they were scrambling to delay interviews, including an appearance on the "Today" show in which his findings would have finally been aired.

The book challenges both popular and scholarly portrayals of Malcolm X, the black nationalist leader, describing a man often subject to doubts about theology, politics and other matters, quite different from the figure of unswerving moral certitude that became an enduring symbol of African-American pride.

It is particularly critical of the celebrated "Autobiography of Malcolm X," now a staple of college reading lists, which was written with Alex Haley and which Mr. Marable described as "fictive." Drawing on diaries, private correspondence and surveillance records to a much greater extent than previous biographies, his book also suggests that the New York City Police Department and the F.B.I. had advance knowledge of Malcolm X's assassination but allowed it to happen and then deliberately bungled the investigation.

"This book gives us a richer, more profound, more complicated and more fully fleshed out Malcolm than we have ever had before," Michael Eric Dyson, the author of "Making Malcolm: The Myth and Meaning of Malcolm X" and a professor of sociology at Georgetown University, said on Thursday. "He's done as thorough and exhaustive a job as has ever been done in piecing together the life and evolution of Malcolm X, rescuing him from both the hagiography of uncritical advocates and the demonization of undeterred critics."

Over the course of a 35- year academic career, Mr. Marable wrote and edited numerous books about African-American politics and history, and remained one of the nation's leading Marxist historians. But the biography is likely to be regarded as his magnum opus. He obtained about 6,000 pages of F.B.I. files on Malcolm X through the Freedom of Information Act, as well as records from the Central Intelligence Agency, State Department and New York district attorney's office. He also interviewed members of Malcolm X's inner circle and security team, as well as others who were present when Malcolm X was shot to death.

Poor health had slowed his progress, but Mr. Marable remained optimistic. "For a quarter-century I have had sarcoidosis, an illness that gradually destroyed my pulmonary functions," he wrote in the volume's acknowledgments. "In the last year in researching this book, I could not travel and I carried oxygen tanks in order to breathe. In July 2010, I received a double lung transplant, and following two months' hospitalization, managed a full recovery." (An interview with The New York Times was planned, but did not take place.)

The book's account of the assassination of Malcolm X, then 39, on Feb. 21, 1965, is likely to be its most incendiary claim. Mr. Marable contends that although Malcolm X embraced mainstream Islam at least two years before his death, law-enforcement authorities continued to see him as a dangerous rabble-rouser.

"They had the mentality of wanting an assassination," Gerry Fulcher, a former New York City police detective who participated in the surveillance of Malcolm X, told Mr. Marable for the book.

That is why "law-enforcement agencies acted with reticence when it came to intervening with Malcolm's fate," the book asserts. "Rather than investigate the threats on his life, they stood back." Paul Browne, a spokesman for the Police Department, did not respond to e-mail and telephone requests for comment this week.

Based on his new material, Mr. Marable concluded that only one of the three men convicted of killing Malcolm X was involved in the assassination, and that the other two were at home that day. The real assassination squad, he writes, had four other members, with connections to the rival Nation of Islam's Newark mosque — two of whom are still alive and have never been charged.

Since Malcolm X's death, the posthumous "Autobiography," along with "Malcolm X," Spike Lee's 1992 film drawn from it, has made a pop-culture hero out of the man who was born Malcolm Little. But the Marable book contradicts and complicates key elements of his life story.

Malcolm X himself contributed to many of the fictions, Mr. Marable argues, by exaggerating, glossing over or omitting important incidents in his life. These episodes include a criminal career far more modest than he claimed, an early homosexual relationship with a white businessman, his mother's confinement in a mental hospital for nearly 25 years and secret meetings with leaders of groups as divergent as the Ku Klux Klan and the Palestine Liberation Organization.

"Malcolm X: A Life of Reinvention" shows, for instance, that at a time when Malcolm X claimed in the autobiography to have "devoted himself to increasingly violent crime" in New York, he was actually in Lansing, Mich., his hometown. Mr. Marable attributes the embroidery of "amateurish attempts at gangsterism" to Malcolm X's wish to demonstrate that the Nation of Islam's gospel of pride and self-respect had the power to redeem even the most depraved criminal.

"In many ways, the published book is more Haley's than its author's," Mr. Marable writes, noting that Haley, who died in 1992, was a liberal Republican and staunch integrationist who held "racial separation and religious extremism in contempt" but was "fascinated by the tortured tale of Malcolm's personal life."

The book maintains that several chapters of the autobiography explaining Malcolm X's evolving but still radical political vision were deleted before publication, perhaps out of Haley's desire to produce a work that "frames his subject firmly within mainstream civil rights respectability at the end of his life." 

The Marable book also sheds new light on Malcolm X's departure from the Nation of Islam and the subsequent feud with the organization and its founder, Elijah Muhammad, preceding his assassination. That split is usually attributed to theological and political differences and the jealousy of Muhammad's children and inner circle.

But Mr. Marable also points to an episode of almost Oedipal sexual duplicity, in which Elijah Muhammad impregnated a woman Malcolm X had loved since he was a young man. "Malcolm must have felt a deep sense of betrayal," Mr. Marable writes.

Malcolm X's subsequent trip to Mecca in 1964 — a likely turning point in his religious evolution — was recounted in both the autobiography and the biopic. The Marable book, however, provides extensive new material about a second, 24-week trip to Africa and the Middle East later that year, drawing on Malcolm X's own travel diary and providing details on a campaign he waged to have the United States condemned for racism in a vote at the United Nations.

As part of that effort to open a foreign front for the civil rights struggle, which was closely monitored by American governmental agencies, Malcolm X met with numerous African heads of state as well as Chinese and Cuban diplomats. The Johnson administration was so upset, Mr. Marable writes, that Nicholas Katzenbach, the acting attorney general, considered prosecuting him for violating a law that bans United States citizens from negotiating with foreign states.

"These are new facts being unveiled, showing just how serious and sustained was Malcolm's interest in the global dimension" of the domestic civil rights struggle, Mr. Dyson said. "They really do suggest he was a subversive figure, trying to undermine the best interests of the U.S. government" in the name of a larger pan-African cause. "That is a fresh insight, one of many."

 Mr. Marable's editor, Wendy Wolf, said Friday evening that "his every fiber was devoted to the completion of this book." She added: "It's heartbreaking he won't be here on publication day with us."