We call it being wise after the event. In the US there is a more vivid expression, "Monday morning quarterbacking". It refers to the conversations people have around office water coolers discussing the previous day's televised American football games, and the performance of the quarterback, the key player who directs the strategy of his team.

This month, as the country pores in anguish over the missed signals that might have warned that the attacks of 11 September were in the offing, there has been enough Monday morning quarterbacking to fill a dozen NFL seasons.

It is a story of tantalising clues and connections, linking global terror and Osama bin Laden to Arab students at US flight schools, of warnings by FBI agents which went unheeded, but which in the light of the subsequent tragedy have taken an aura of exact prophecy. It is also a human story: had not John O'Neill, the FBI's most experienced counter-terrorism official and expert on Mr bin Laden and al-Qa'ida, not been involved in the quarrel with the Bureau that would see him resign in August 2001, might these warnings have been more vigorously pursued?

In retrospect, there were indications in abundance, stretching back seven years, that Islamic religious terrorists intended to hijack planes and crash them into important buildings, and were boning up on how to do so. Whether these pointers were enough to have tipped off the US authorities to what was to happen is quite another matter.

As long ago as December 1994, French security forces foiled a plot by Algerian terrorists to ram a commercial airliner into the Eiffel Tower. The following year, plans for a similar operation against the CIA headquarters in Langley, Virginia, came to light in the Philippines and were confirmed by Ramzi Yusef, one of those convicted for the first attack on the World Trade Centre.

Since 1999 a National Intelligence Council report, with input from the CIA, was available on the internet to anyone who cared to look. It warned that terrorists might "crash-land an aircraft packed with high explosive into the Pentagon, the CIA or the White House", in retaliation for the August 1998 US missile attacks on Mr bin Laden's camps in Afghanistan.

These warnings however were generic. From early 2001 the clues, albeit fragmentary, became more specific, both at home and from abroad. By late June, George Tenet, the CIA director, told the White House it was "highly Likely" that al-Qa'ida would carry out an attack "in the near future", perhaps over the 4 July holiday. But nothing happened, either in the US or against American installations overseas. Over the following weeks, the alarm subsided.

But at that very moment, on 10 July, Kenneth Williams, an agent at the FBI's Phoenix office, sent a memo to the Bureau's headquarters, which was copied to John O'Neill in New York. Noting that an unusual number of Middle Eastern men were training at Arizona flight schools, the memo speculated they could be members of an al-Qa'ida operation, and mentioned Mr bin Laden by name. It is said however to focus on one student, Zakaria Mustapha Soubra who was linked to al-Qa'ida and who had been under investigation by the Bureau for 12 months already. Mr Williams recommended the FBI check flight schools across the country for other students who might have terrorist links. But nothing was done. O'Neill was feuding with his bosses, while Robert Mueller, the FBI director, apparently was not told about the memo until just after the attacks.

On 16 August 2001 came the last and most glaring red flag, with the arrest of Zacarias Moussaoui, widely believed to be the "20th hijacker" at a Minnesota flight school after his trainers tipped off the FBI about his strange fixation on steering a plane, and lack of interest in how to take off or land it. Again no action was taken, and recommendations that Mr Moussaoui's computer files be examined were ignored. Coleen Rowley, another FBI agent in the Minneapolis office has now written a blistering memo to a Congressional Committee claiming that, contrary to the assertions of Mr Mueller, the FBI not only had a host of warning signs on which it should have acted - but also rejected urgent demands for an aggressive investigation of Moussaoui before 11 September.

So frustrated were agents in the Minnesota field office, she says, that they sidestepped their superiors and took the case to the CIA. For this breach in protocol, they were roundly scolded by headquarters in Washington. This bureaucratic spat highlights one of the more profound reasons for arguably the most colossal intelligence failure since Stalin's refusal in May and June 1941 to heed the evidence that Hitler was about to invade the Soviet Union. For 11 September exposed the institutional, even psychological, obstacles that prevented the clues being acted upon.

For years the US security apparatus has been bedevilled by chronic rivalry between the CIA, reponsible for foreign espionage operations, and the FBI which handles domestic counter-intelligence. These turf battles explained in part why Aldrich Ames, surely the most detectable mole in espionage history, survived and flourished for so long. And the lack of inter-agency co-operation played a role in the inability of the US authorities to "connect the dots" to get a more accurate take on what would happen on 11 September.

Even more fundamentally, Americans probably don't like spying very much. It is a messy, intuitive, imprecise business, at odds with the American yearning for the exact, factual truth. That is surely why the US has placed its faith in hi-tech intelligence reconnaissance, electronic eavesdropping and the like. But the abundance � often indigestible surfeit � of "sigint" or signals intelligence cannot make up for the lack of human intelligence; of spies on the ground reporting not on what the enemy is doing at the time but, far more important, on what he intends to do, and of analysts who can "think outside the box". At one point in the aftermath of 11 September, Mr Tenet was asked at a Congressional committee hearing how it was that John Walker Lindh, the so-called "American Taliban" had managed to get close to Osama bin Laden and the al-Qa-ida high command, and the CIA had not. Mr Tenet bristled, but the question was only slightly unfair. Only now has the US truly grasped the importance of cultivating its own penetration agents, even though it has had a natural pool of Arab-Americans, Afghan-Americans and the like on which to draw.

But in late summer 2001 the threat was still theoretical. On 6 August, President Bush received a one-and-a-half page daily intelligence briefing, which included the suggestion Mr bin Laden might hijack planes � but with no hint of how, where, or when. "Bush Knew" screamed the front page of the New York Post recently.

He didn't. But the FBI did know something. The question now is whether, armed with that knowledge, it did enough to call the accumulation of clues, a "smoking gun" is probably an exaggeration. None, for instance, of the eight men reportedly named by Mr Williams, were among the 19 hijackers of 11 September 2001. Nothing gave details of how that day flights AA 11 and UA 175, laden with jet fuel would smash into the twin towers of the World Trade Centre and cause them to collapse. Among those killed was the centre's new head of security John O'Neill.