It started the day after the attacks on the twin towers, with the discovery of a flight manual in Arabic and a copy of the Koran in a car hired by Mohammed Atta and abandoned at Boston airport. In the immediate shocked aftermath of the attacks, these findings were somehow reassuring: American intelligence was on the case, the perpetrators were no longer faceless.

In less than a week came another find, two blocks away from the twin towers, in the shape of Atta's passport. We had all seen the blizzard of paper rain down from the towers, but the idea that Atta's passport had escaped from that inferno unsinged would have tested the credulity of the staunchest supporter of the FBI's crackdown on terrorism.

Yet we were still in the infancy of coincidence. On September 24 the belongings of alleged terrorist Zacarias Moussaoui threw up a cropdusting manual, while four days later came Atta's suicide note, the one with the counsel to shine your shoes before you meet your maker - a piece of advice which seemed suspiciously Norman Rockwellesque. It was here, too, that the stuff about 72 virgins awaiting him in heaven first started to circulate.

In December the laughing, boasting video of Osama bin Laden was unearthed in a house in Jalalabad. The new year saw no let-up in this serendipitous trove - January turned up an email sent by "shoe bomber" Richard Reid from a Paris cybercafe (and found on its hard disk) shortly before boarding the Paris-Miami flight in which he claimed responsibility in advance for downing the plane. (Luckily or carelessly, depending on your perspective, Reid had pocketed a business card from the cybercafe.)

And then, last Friday, Major General Frank Hagenbeck revealed that Americans had found a whole shelf of field manuals on undertaking terrorist activity, to put beside the instruction manual on how to use light automatic weapons left in a training camp in January.

Apart from the fact that the al-Qaida network seem to have a catastrophic way with lost property, isn't it strange that these most demonised and potent of terrorists seem unable to operate any weapons without a manual? Dad's Army is nothing - this bunch sounds as if they wouldn't be able to programme the video. And if the quality of their manuals is anything like those most of us have come across, they will still be wrestling with them long after the guarantee has run out.

Of course you could interpret these discoveries differently. You could detect in them the clear hand of American propaganda. This isn't, of course, to claim a dirty tricks department somewhere in the heart of Washington. That would have you immediately accused of peddling conspiracy theories, though I'm coming to think that conspiracy theories have had a bad press. What are they, after all, but "joined-up government" by another name?

All these discoveries can't obscure four things that American intelligence agencies have notably failed to find. First, even with a bloated expenditure exceeding Russia's total defence budget, they never managed to find out about September 11 before the event. Rhodri Jeffreys-Jones's new book, Cloak and Dagger: A History of American Secret Intelligence (Yale), shows how, almost since their 19th-century inception, American intelligence bureaux have invented or exaggerated a succession of menaces to defend their spiralling budgets and demonstrate their own usefulness while failing to tackle effectively other, more substantial threats.

Second, despite a reward of $2.5m offered at the end of January, the FBI still hasn't discovered those responsible for last year's anthrax attacks.

Third, American intelligence, tragically, didn't find Daniel Pearl, the US journalist kidnapped and murdered in Pakistan.

Fourth - and most spectacular - despite having highly sophisticated satellite tracking equipment, and offering a reward of $25m for information leading directly to his apprehension or conviction, they still haven't found Bin Laden.

Is this one reason why the US is talking about an attack on Iraq - a flexing of the military biceps to distract from flabby intelligence? Whatever the case, to find one training manual might be regarded as a stroke of luck. To find a shelf-full looks like desperation.