It is hard to imagine how intelligence and law enforcement agents could close down the money network that sustains Osama bin Laden's terrorist group.

Cash is frequently transported in dollar bills inside suitcases and bags, often in small amounts that would not raise suspicions even if they were intercepted.

However, even this apparently underground cash rises to the surface of the traditional banking system at some point in its journey across the world. It is at that moment that the international authorities hope to trace and - if possible - seize it.

US officials have well rehearsed investigative techniques for following the money in white collar crimes such as fraud and money laundering. Their usual tactic is to collect a paper trail of financial records using search warrants on individuals or businesses, or by grand jury subpoenas served on financial services companies.

The FBI has already passed a list of the names of the 19 suspected hijackers - plus two other suspect individuals - to all US banks via regulators at the Federal Reserve and the Office of the Comptroller of the Currency in Washington. The banks have been asked to check their records for "any relationships or transactions" with the suspects.

However, they have little experience of applying such straightforward methods to terrorism cases. "I have never worked on a terrorism investigation in my life," said one senior federal government financial investigator with 24 years of service.

In the US, the effort is being managed by Treasury officials in Washington in a new group called the Foreign Terrorist Asset Tracking Center. The centre has been created in the hope of avoiding the turf wars between rival US and international agencies that has prevented the sharing of vital information on suspected and proven terrorists in the past.

One of the new group's key players is the Financial Crimes Enforcement Network. FinCEN is trawling through bank databases to cross-reference bank records with the names of known terrorists as well as those involved in the attacks on New York and Washington.

Paul O'Neill, Treasury secretary, told senators last week: "We for the first time will systematically use all the information - intelligence assets - that are at the disposal not only of our government, but co-operating governments around the world to begin a closing down of bank accounts, asking other governments to block accounts, and where there is enough evidence to do it quickly, to confiscate amounts that are in these accounts."

The investigation may come up against existing banking regulations. In many states, banks are required by law to report suspicious transactions and to verify the credentials of parties involved in transactions over a certain sum.

However, in most jurisdictions - including two leading banking centres, the UK and Switzerland - police can only obtain details of specific named accounts under suspicion. "They are not allowed to go on a fishing expedition through a bank's entire records in the hope of turning something up," said a UK lawyer involved in money-laundering investigations.

A further difficulty for investigators is that the US banking system is immensely porous. One of the biggest holes in the system is the business of correspondent banking, in which some of the largest international banks offer services to foreign institutions. According to a study led by Senator Carl Levin and his staff on the Senate's permanent sub-committee on investigations, the system has allowed "high risk foreign banks" widespread access to the US.

Moreover, according to the Bankers' Almanac, the banking industry directory, several established banks even have correspondent relationships with the Afghan central bank, in spite of the pariah status of the Taliban regime.

Da Afghanistan Bank, which also performs commercial bank functions through branches across the country, has correspondent relationships with Bank of New York, Chase Manhattan, Citibank and American Express. In London, the bank has relationships with HSBC and National Westminster, according to the Bankers' Almanac.

Additional reporting by Harvey Morris in London and William Hall in Zurich