In the wake of those attacks, President Bush and the F.B.I. issued a list of the world's 22 most wanted terrorists. Sudan has kept files on many of them for years.

From the autumn of 1996 until just weeks before the 2001 attacks, the Sudanese government made numerous efforts to share this information with the United States all of which were rebuffed. On several occasions, senior agents at the F.B.I. wished to accept these offers, but were apparently overruled by President Clinton's secretary of state, Madeleine Albright, and her assistant secretary for Africa, Susan Rice, both of whom would not comment for this story after repeated requests for interviews. Vanity Fair has obtained letters and secret memorandums that document these approaches. They were made directly to the State Department and the F.B.I., and also via a series of well-connected U.S. citizens who tried to warn America that the Sudanese offers were serious and significant.

By definition, September 11 was an intelligence failure. As the C.I.A. man puts It, We didn't know it was going to happen." Some of the reasons for that failure were structural, systemic: the shortage of Arabic-speaking agents, the inability of C.I.A. officers to go underground in Afghanistan.

This one was more specific. CE Had U.S. agencies examined the AF Mukhabarat files when they first REI had the chance in 1996, the prospects of preventing al-Qaeda's subsequent attacks would have been much greater. Tim Carney, the last U.S. ambassador to Sudan, whose posting ended in 1997, says: "The fact is, they were opening the doors, and we weren't taking them up on it. The U.S. failed to reciprocate Sudan's willingness to engage us on serious questions of terrorism. We can speculate that this failure had serious implications-at the least for what happened at the U.S. Embassies in 1998. In any case, the U.S. lost access to a mine of material on bin Laden and his organization."

How could this have happened? The simple answer is that the Clinton administration had accused Sudan of sponsoring terrorism, and refused to believe that anything it did to prove its bona fides could be genuine. At the same time, perceptions in Washington were influenced by C.I.A. reports that were wildly inaccurate, some the result of deliberate disinformation. The problem, Carney says, was "inadequate vetting and analysis by the C.I.A. of its own product." That, in turn, was being conditioned by the Clinton administration's hostility to Sudan's Islamic regime: "Despite dissent from the State Department's own Bureau of Intelligence and Research, U.S. intelligence failed be- cause it became politicized."

Osama bin Laden, his four wives, his children, and numerous "Afghan Arab" followers who had helped drive the Soviets from Afghanistan went to Sudan from Saudi Arabia early in 1991. They chose Sudan for two main reasons. First, the restless, radicalised veterans of the Afghan war were unwelcome in most Arab countries, but Sudan left its doors open. Second, bin Laden liked Sudan's politics. The Islamic radicalism of the government's then ideological leader, the philosopher Hassan aI-Turabi, who had come to power in a coup d'etat in 1989, was at its bracing zenith. The Sudanese, in turn, welcomed bin Laden as an investor. His family had built most of Saudi Arabia's infrastructure, and they saw his wealth and experience as an engineer as valuable resources in developing Sudan.

"If we'd had communication with the U.S., we could have been on the same wavelength. We could have exchanged notes." An foreigners in Sudan were subject to some degree of surveillance. Disclosure of bin Laden's link with Egyptian Islamic Ji-had led the Mukhabarat to watch him and his Afghan Arab followers more closely. Lieutenant General Gutbi al-Mahdi, Mukhabarat director general from 1997 until 2000, says the service started keeping tabs on "the entire bin Laden clique. ...We had a lot of information: who they are, who are their families, what is their education. We knew what they were doing in the country, what is their relationship with Osama bin Laden. And photographs of all them." Not long into the 1990s, Sudan's Islamic fervor was already being tempered by pragmatism. Desperate for investment, especially to develop its vast reserves of oil, the government submitted to the stringent economic medicine prescribed by the World Bank, slashing inflation and privatizing state-owned industries. (Osama bin Laden himself became the Sudan agent for the British firm Hunting Surveys, which plays a large role in oil prospecting and whose military division makes about a fifth of the West's Trident nuclear missiles.) In 1994 it tried to assert its anti-terrorist credentials by assisting France in the capture of Illich Ramirez Sanchez, better known as "Carlos the Jackal," the notorious Venezuelan-born terrorist who claims to have killed 83 people, now serving a life sentence in France.

The U.S., however, remained convinced that Sudan was sponsoring terrorism. Toward the end of 1995, the then U.S. ambassador, Don Petterson, was instructed to deliver an un-signed, secret note to the spiritual leader, Hassan al-Turabi, and President Omar al-Bashir. It said the U.S. was "aware of Sudan's involvement in terrorist plots against us," and warned that if such a plot came to fruition there would be a harsh reaction. It could result in "the international isolation of Sudan, in the destruction of your economy, and in military measures that would make you pay a high price."