The questions were asked all over America and much of the rest of the world on September 11, 2001: How could this have happened? Who are these people? Why do they hate America so much? What do they hope to gain by slaughtering innocent people?

Lawrence Wright, who taught for two years at the American University in Cairo, has done the best job yet of providing answers. His book is a searching examination of the lives and personalities of the main players, Americans as well as Arabs; a study of the political and religious ideas that motivated them, and a tale of a globe-spanning cat-and-mouse game between terrorists and those whose job it was to thwart them. His cast of characters is of Dickensian size and Dostoyevskyan complexity; the list of principal figures in his endnotes bears 86 names.

Among the Arabs, Wright focuses on Osama bin Laden and his chief deputy, Ayman al-Zawahiri. The reader gets a chilling sense of the religion-based fanaticism that drives them both and still threatens the world today. The most interesting of his American portraits is of John O'Neill, a spy-hunter who may be fairly described as a driven fanatic of a different sort, a handsome womanizer turned Christian religious zealot who, by a weird twist of fate, died in the World Trade Center on September 11th.

Wright makes clear that the driving force behind bin Laden and Al Qaeda has two mainsprings: the fundamentalist conviction that Islam is the only true religion and that all who reject it or follow other religions are enemies of God and must be eliminated; and a pervasive resentment that the decadent, infidel west has so far outstripped the once-dominant Islamic world in science, living standards, the arts and civilization in general. America is indeed the "great Satan" to the Islamic zealot, the heart and soul of opposition to all that they hold sacred.

Wright also makes clear that the main reason Islamic nations are so mired in backwardness is the suffocating grip of their religion on daily life. Theirs is a world, in Wright's words, where "reality knelt before faith."

It is clear from Wright's research that the fundamentalists who created Al Qaeda are extremists who have twisted many clear tenets of the Koran to their own sinister uses --- e.g., the Koran's explicit prohibitions on suicide and on the killing of noncombatants. Their leaders decreed, for instance, that anyone who rejects or opposes Islam, even innocent bystanders, is ipso facto an apostate and should be killed, and that suicide bombers are glorious martyrs for the cause rather than sinners. To the western reader, this is chilling stuff. To moderate Moslems it is doubtless heresy.

Shifting to the American side of the drama, Wright's major emphasis is on the paralyzing interagency wars, idiotic rules and personality conflicts that prevented the sharing of vital information about the 9/11 conspiracy among the FBI, the CIA and the National Security Agency. Time after time he cites cases where important links in the chain of evidence were deliberately withheld from people who wanted and needed them and who might have been able to use them to thwart the developing tragedy. It is a tale not just of bureaucratic bungling, but of willful refusal to cooperate that resulted in the deaths of 2,749 innocent people. The reader does not know whether to explode in rage or simply weep.

The scope of Wright's research is vast. He delves deeply into the lives and writings of fundamentalist Islamic scholars like Sayyid Qutb, who preceded the rise of Al Qaeda. He explores the movement's roots in resistance to the Soviet invasion of Afghanistan and its nonstop rage at the United States for our support of Israel. Some of the points he makes have been made before, by scholars like Bernard Lewis, and the tale of American bureaucratic turf wars was told in the U.S. Senate's 9/11 commission report. But seldom have all the threads been brought together with such cogency and urgency as in this book.

There is one other theme that lurks behind Wright's text without being really emphasized: the danger posed by fanaticism of any sort --- religious, political, personal. Eric Hoffer sounded that note in his 1950 classic THE TRUE BELIEVER. Wright never states it outright, but it is a clear subtext of his masterly and important book.