How can the wall of silence be cracked?

Central to the investigation is establishing the money trail. The authorities are anxious to nail the mastermind, the person who supplied the cash, who paid the hijackers through flying school. Here, they fear they will come up, eventually, against a centuries-old practice of criminals in the Middle East and Asia transferring cash without leaving any evidence.

Central to the investigation is establishing the money trail. The authorities are anxious to nail the mastermind, the person who supplied the cash, who paid the hijackers through flying school. Here, they fear they will come up, eventually, against a centuries-old practice of criminals in the Middle East and Asia transferring cash without leaving any evidence.

In the West it is called "underground banking"; in India, where it began, it is known as "hawala". You want to send money from A to B. Instead of going through a normal bank and using a telegraphic transfer that can be traced, you contact a hawala banker. He takes your cash at A and contacts a counterpart at B. The counterpart pays over the same sum in cash to someone nominated by you. It is based entirely on trust, is totally discreet and is virtually impenetrable.

The biggest difficulty for the authorities in this case is the degree of planning. This was not a hurried crime, conceived on the spur of the moment. The preparations go way back, possibly several years, when at least some of the hijackers may have been in training camps in Afghanistan. Much of the detail may have been thrashed through, not in the US, but overseas, with Germany, where several of the hijackers had connections, being the likely base.

Critically, for the investigators, they do not have an informer. Despite all the sophisticated techniques the FBI can call upon, they do not have the one thing they would love more than any other: someone they can lean upon to plea-bargain, to seek a lighter sentence in return for testifying. Much is made in the US of the relative success of the investigation into the bombings of the embassies in Kenya and Tanzania. But the authorities were only able to get anywhere because, at the last moment, one of the suicide bombers lost his nerve and jumped from a truck. He was caught and helped the inquiry.

It is hard to under-estimate the importance of this. Not one of the hijackers cracked, not one of their associates, fearing imminent arrest, has so far come forward. This was a well-drilled, well-prepared group, loyal to each other and to their cause.

In his speech to Congress last week, President Bush likened the international terrorist network to the Mafia. In terms of global reach it is a fair comparison. But that is all. The network behind the US suicide attacks may bear the hallmarks of the Mafia but has none of the weaknesses.

In the past, Mafia bosses have been trapped by informers. The Italian crime organisation involves itself in lots of activities from drugs to loan-sharking, all of which increase the risk of being caught; and many of its senior members are well-known to the police, making penetration and surveillance easier.

For all that, progress is being made – although not at the speed Washington might wish. It is noticeable how much slower the investigation has become, as clues and identities are double-checked to avoid further embarrassments and red herrings.