“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.