Startling claims in Newsweek magazine that the Central Intelligence Agency (CIA) knew of and tracked two of the September 11th hijackers to and from the US almost two years before the attacks are likely to add explosive fuel to congressional claims of a massive US intelligence failure.
And, while the embattled FBI has been centre stage in recent weeks, the new allegations are certain to point an uncomfortable light on the external intelligence agency's extraordinary omissions and to raise for the first time in a really credible way the question of whether the attacks could have been foreseen and prevented.
Their emergence, a week ahead of key congressional hearings, must also raise the question of whether the two agencies are engaged in a secret propaganda war - against each other.
The Newsweek report says that the CIA knew in advance of a "summit meeting" of al-Qaeda in January 2000 and of the attendance there of two men then living in the US, Nawaf Alhazmi and Khalid Almihdhar.
The agency sought the help of the Malaysian security services in monitoring the attendance at the meeting and a few days later tracked Alhazmi as he flew back to Los Angeles.
Agents later discovered that Almidhar flew with him on a multiple-entry visa which would later be renewed by a State Department consular officer in June 2001, despite the fact that the CIA had linked him to the bombing of the USS Cole in October 2000.
Quite simply the CIA, which did not have the legal right to continue the surveillance in the US, had failed to tell either the FBI or the Immigration and Naturalisation Service or the State Department of the presence of the two men on US soil.
The FBI would not know of their presence until three weeks before the attacks, after the CIA director, Mr George Tenet, alarmed by intelligence of an imminent al-Qaeda attack, ordered the review of all related files.
On August 23rd, 2001, the CIA sent out an all-points bulletin, launching FBI agents on a frantic and futile search for the two men. On September 11th the two walked on to AA Flight 77 and crashed it into the Pentagon.
During the year and nine months after the CIA identified them as terrorists both men lived openly in the US under their real names, opening bank accounts, enrolling in flight schools, and even obtaining driving licenses.
Finding them would not have been difficult, the magazine reports: "Agents might have used another resource to pinpoint their location: the phone book. Page 13 of the 2000-2001 Pacific Bell White Pages contains a listing for 'Alhazmi Nawaf M 6401 Mount Ada Rd. 858-279-5919'."
Had the CIA and the FBI overcome their now-notorious reluctance to speak to each other, could someone have connected the dots?
The benefit of hindsight makes it easy to say yes, say defenders of the agencies, but the fact remains that they have real questions to answer.
Surveillance of the two men while in the US would have revealed their interest in flight lessons and on at least one occasion would have led them to Mohamed Atta, the man believed to have commanded the September 11th operation, as well as other hijackers.
"The links would not have been difficult to make," Newsweek claims.
"Alhazmi met up with Hani Hanjour, the Flight 77 pilot, in Phoenix in late 2000; six months later, in May 2001, the two men showed up in New Jersey and opened shared bank accounts with two other plotters, Ahmed Alghamdi and Majed Moqed.
"The next month, Alhazmi helped two other hijackers, Salem Alhazmi [his brother\] and Abdulaziz Alomari, open their own bank accounts.
"Two months after that, in August 2001, the trail would have led to the plot's ringleader, Mohamed Atta, who had bought plane tickets for Moqed and Alomari. What's more, at least several of the hijackers had travelled to Las Vegas for a meeting in summer 2001, just weeks before the attacks."
And had the CIA maintained surveillance on the flat in Kuala Lumpur where al-Qaeda met it would have come across the alleged 20th hijacker, Zacarias Moussaoui, now awaiting trial in Virginia for his alleged role in the plot.
"There's no question we could have tied all 19 hijackers together," one anonymous FBI official told the magazine.
Yesterday, the administration was going into damage limitation mode - a senior official reminded journalists that the events started to unfold under President Clinton's watch and urged journalists not to rush to judgment ahead of congressional reports. But the White House can be severely hurt by the fact the story has taken until now to come to light.
Mr Tenet also faces serious questions. In February he told a Senate panel that he was "proud" of the CIA's record. He insisted that the terrorist strikes were not due to a "failure of attention, and discipline, and focus, and consistent effort".
