By ANDREW GUMBEL
President George W. Bush's Administration was warned repeatedly that a devastating attack on the United States was on its way, including a State Department advisory as late as September 7.
But it either failed to read the signals or was unable to follow up on intelligence tips in time to prevent last week's onslaught on the country.
Evidence has surfaced in the past few days suggesting that the "colossal failure of intelligence" described by several senior politicians was not entirely the result of ignorance.
In the three weeks before the attack, the FBI was looking for two suspected associates of Osama bin Laden who turned out to be among the 19 suicide hijackers.
That search was part of a pattern of alarm signals sounding in intelligence circles from late August.
Around the time the FBI was receiving its tip-off about the two men - one of whom was wanted for another suspected bin Laden operation, the bombing of the USS Cole in Aden harbour last October - security was abruptly heightened at the World Trade Center.
Sniffer dogs were introduced along with systematic checks on trucks bringing in deliveries. No explanation has been given for these measures.
Also in late August, bin Laden boasted in an interview with the London-based newspaper al-Quds al-Arabi that he was planning an unprecedentedly large strike against the US.
Then, four days before calamity struck, the State Department sent out a worldwide advisory, repeating warnings it first made in May that "American citizens may be the target of a terrorist threat from extremist groups with links to Osama bin Laden's al Qaeda organisation".
The reason for the advisory was a heightened concern over military bases in Japan or Korea.
Although these were clearly not the targets most at risk, the very fact that the State Department chose to put out an advisory was a sign that "something was cooking", says former Secretary of State George Shultz.
"I have no idea what intelligence lies behind the warning," Mr Shultz said, "but they put this out because they had some sort of intelligence."
There is nothing new about the fear that bin Laden and his organisation might strike against the United States at any time. Particularly since the simultaneous bombing of the US embassies in Kenya and Tanzania in August 1998, a crime the United States has laid squarely at bin Laden's door, repeated alerts have led to the temporary evacuation of American Government buildings overseas.
Aside from the general threat, however, there seems to have been some misreading of the evidence at hand.
Over the weekend, for example, FBI director Robert Mueller said he had been taken entirely by surprise by the revelation that a number of the hijackers had received pilot training in the US.
"If we had understood that to be the case, we would have, perhaps one could have, averted this," he said.
And yet, during the trial this year of four defendants charged with involvement in the 1998 embassy bombings, it emerged that two suspected contacts of bin Laden's, Essam al Ridi and Ihab Ali Nawawi, had received pilot training in Texas and Oklahoma.
That would suggest, contrary to Mr Mueller's statement, that the FBI had solid information about the pilot-training scheme for three years.
There is also mounting evidence that the Bush Administration, preoccupied with its "Son of Star Wars" missile defence system to guard against rogue nuclear, biological or chemical attacks from overseas, did not give sufficient weight to the demonstrably more pressing threat of low-tech guerrilla attacks.
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