The accusatory memo from a respected Minneapolis agent, Coleen Rowley, to the FBI director, Robert Mueller, is a challenge to the Bush administration's claims that the evidence available before September 11 had not been enough to forestall the attack.

Mrs Rowley argues that not only did FBI headquarters fail to assist a crucial Minneapolis investigation, a supervisory agent in Washington actually intervened to suppress it.

She has appealed for protection from disciplinary action under a federal whistleblower law, and is likely to become a key witness in the unfolding inquiry into America's greatest intelligence failure since Pearl Harbour.

The letter, which the aggrieved agent said came "from the heart" stops short of alleging a "cover-up" as "too strong a characterisation". But it does directly accuse Mr Mueller and his aides of "shading" and "skewing' the facts to avoid "personal and/or institutional embarrassment" or for political reasons.

The memo, published on the Time magazine website, focuses on the investigation of Zacarias Moussaoui, a French Moroccan facing trial for involvement in the September 11 conspiracy. He was arrested on August 15 last year after instructors at a flight school became suspicious.

Hasty research into Mr Moussaoui's background uncovered French intelligence suspicions that he had links to Islamic extremist groups, and Mrs Rowley approved a request for a search of Mr Moussaoui's computer and personal effects, but the request met with scepticism in Washington, and by one unnamed supervisory special agent (SSA) in particular.

"Why would an FBI agent(s) deliberately sabotage a case?" Mrs Rowley asks rhetorically, before arguing that the FBI culture punishes mistakes more than inactivity. She wrote: "In most cases avoidance of all "unnecessary" actions/decisions by FBI HQ managers (and maybe to some extent field managers as well) has, in recent years, been seen as the safest FBI career course."

After September 11, Mr Mueller told Congress that there had been no signs that such an attack was being planned. When it emerged there had been clues, he said there had not been enough information to stop the plot.

Mrs Rowley contradicts that assertion. "Although I agree that it's very doubtful that the full scope of the tragedy could have been prevented, it's at least possible we could have gotten lucky and uncovered one or two more of the terrorists in flight training... "

The FBI also came under attack yesterday in an article in the New Yorker, which quotes law enforcement officials as saying that the 19 September 11 hijackers failed to take basic precautions to avoid detection.

The FBI refused comment on both the New Yorker article and the Rowley memorandum, which it has declared classified.

What the memo says

Extracts from FBI agent Coleen Rowley's memo to FBI director, Robert Mueller, May 21 2002

I have deep concerns that a delicate and subtle shading/skewing of facts by you and others at the highest levels of FBI management has occurred and is occurring.

I feel certain facts have been omitted, downplayed, glossed over and/or mis-characterised in an effort to avoid or minimise personal and/or institutional embarrassment on the part of the FBI and/or perhaps even for improper political reasons.

It is obvious that the agents in Minneapolis... did fully appreciate the terrorist risk/danger posed by Moussaoui and his possible co-conspirators even prior to September 11... So I think it's very hard for the FBI to offer the "20-20 hindsight" justification for its failure to act!

The FBI supervisory special agent (SSA) most involved in the Moussaoui matter... seemed to have been consistently, almost deliberately thwarting the Minneapolis agents' efforts...

The fact is that key FBI HQ personnel... continued to, almost inexplicably, throw up roadblocks and undermine Minneapolis' by now desperate efforts to obtain a search warrant, long after the French intelligence service provided its information.