Early on the morning of February 26 1993, a bright yellow Ford Econoline rental van glided down a ramp to an underground car park and slipped unnoticed into a space beneath the New York World Trade Center.

A cheap cigarette lighter was used to ignite four 20ft-long fuses. Two men inside drove away in a waiting car. Twelve minutes later 1,200lbs of explosives and several tanks of hydrogen exploded. Islamist terrorism had hit the US.

The symbolism was lost on one of the bombers, Mohammed Salameh. What most concerned him was that he should be able to reclaim the $400 deposit he had paid when he hired the van from an office of Ryder truck rental. Money would lead him to jail, as it would his fellow bomber, Ramzi Yousef.

The first clue for the FBI investigators put on the trail of the bombers, was Salameh's return to the DIB Leasing Agency at 1558 Kennedy Boulevard, Jersey City to reclaim the deposit. He needed the money to buy an air ticket out of the US. Starting with this first clue, investigators tracked down both men.

US investigators established that Yousef was financed by donations from a company importing Holy Water from Mecca into Pakistan. Ever since, the money trail leading to the sources of terrorist financing have been like a scent for investigators to follow.

What has emerged is that the social and financial network of charities, personal donations, businesses and money set aside by the considerable number of rich who sympathise with attacks on the west and which sustained and encouraged Salameh and Yousef, is broadly similar to the network which has sustained al-Qaeda.

These sources of finance are expected to sustain Osama bin Laden's al-Qaeda network, whether or not he survives the manhunt for him in Afghanistan.

A trail of cut-price rental cars, accounts of an argument over small change with a bartender in Portland, Maine, and evidence of a modest amount spent on a "last meal" in a pizza parlour bear poignant testimony to the discipline of the September 11 hijackers.

But these logistical aspects to the plot barely betray the size of al-Qaeda's vast pool of resources.

Of far greater importance are the enormous sums which have for more than two decades been pumped into the financial network available to terrorists ranging from the World Trade Center bombers in 1993 to the al-Qaeda hijackers who razed it to the ground eight years later.

Bin Laden's al-Qaeda organisation became the most formidable group of its kind. But the man who inherited $300m at the age of 10 on the death of his father - as his share of what is today a $36bn family business, the Saudi Binladin Group - is none the less unable to finance this global campaign on his own.

In Afghanistan during the 1980s, bin Laden had been footing the bill for Arab fighters in the anti-Soviet war at up to $25,000 a month, according to those who knew him then. By any calculation his personal fortune would have been rapidly depleted, requiring that he have access to new funds. As a result, since those days he has broadened and deepened his sources of finance.

According to several detailed studies, Islamist organisations - many of them linked to bin Laden - can today draw on funds estimated at between $5bn and $16bn. Much of this has been donated, particularly from Saudis and from Kuwait, source of millions a month.

How much of this reaches al-Qaeda is uncertain. Until the US launched its attacks on Afghanistan last month, the Taliban was estimated by the CIA to be earning a large share of the proceeds from the country's $6.5bn-$10bn a year narcotics trade, limiting its needs from other sources.

A study of bin Laden and other Islamic terrorists by Simon Reeve, a London journalist, claims that by early 1999 bin Laden was taking a cut of up to 10 per cent from the trade, giving him an income of up to $1bn.

How much this income matters is a source of much debate, however. Reeve notes: "By late 1998 and early 1999 the size of bin Laden's bank balance had become almost an irrelevance. He no longer needed to be rich because many of al-Qaeda's cells were virtually self-sufficient. His soldiers were working for reward from Allah, not for financial gain. Bin Laden himself was required to pay little more than initial start-up costs and the price of an occasional air fare."

To date, most of al-Qaeda's legitimate business ventures - the most extensive having been in Sudan - have been failures. To meet the needs of his global agenda, bin Laden has therefore turned for finance to the same world that nurtured his political beliefs and religious extremism.

"A lot of his money comes from disgruntled Saudi merchants," says Jean-Francois Seznec, a former Gulf banker who lectures on Arab politics and finance at Georgetown university in Washington DC. "A 'mule' will tour sympathetic merchants in Jeddah, collecting $5,000 from each. They never give more than $5,000 a time so you have to go and see them regularly," he said.

For many Saudis support for such charities is an integral part of daily life, a pillar of Islam being the requirement to give zakat, charitable donations, calculated at 2 per cent of a person's income.

The potential offered by charities - as sources of finance, as a network through which propaganda can be disseminated and as means of enhancing one's reputation among Muslims - was not lost on al-Qaeda.

During his testimony at the trial which found him guilty of the 1998 bombings of the US embassies in Nairobi and Tanzania, L'Houssaine Khertchou said that while he was in Nairobi, al-Qaeda members there were "dealing with" Mercy International Relief Agency.

He said that Osama bin Laden and Muhammad Atef, al-Qaeda's military training chief who was killed in Afghanistan during a US bombing raid two weeks ago, were linked to the agency. Bin Laden, he said, was given an identity card by the agency, which was registered with the Kenyan authorities, whose lax processing allowed easy registration of charities and personnel and gave bin Laden a cover.

The US prosecutors told the court that Mercy International had a legitimate charitable purpose, but it had other purposes "contrary to that". The court was told that bin Laden's former private secretary, Wadih el-Hage, also registered a charity in Nairobi called Help Africa People.

El-Hage, who was tried in New York for the embassy bombings and later jailed, said Mercy International was financed by "Saudi merchants in Saudi Arabia". The same charity also employed in its Pakistan operation relatives of Ramzi Yousef, mastermind of the 1993 bombing of the New York World Trade Center, with whom al-Qaeda has had ties, according to Simon Reeve's investigation.

Another of the African embassy bombers, Ahmed el-Fadl, said in court he had worked for an al-Qaeda member who was behind another charity, the Qatar Charitable Foundation.

"The guy, he runs a group, he is one of our membership, one of the al-Qaeda group membership, and also he is [Sudan's] Islamic National Front membership, and he was in Afghanistan," al-Fadl said. "So he helped our people for the travel, documents, and also if some money comes from the Gulf area to the organisation, he gives the group some money from that money."

Of these funds, $20,000 was passed to a Sudan-based Eritrean Islamist group which was supported by al-Qaeda and which was planning attacks in Eritrea, al-Fadl said.

One of the Nairobi bombers, Harun Fazil, also travelled often to Somalia on behalf of Help Africa People, according to trial testimony. He used funds raised overtly in areas such as the Gulf to implement an anti-malaria project there.

At the same time an Afghanistan-trained member of the team which would later bomb the US embassy in Tanzania, Khalfan Khamis Mohamed, was travelling by fishing boat from Kenya to Somalia to give military training to an al-Qaeda backed Islamist group, suggesting that al-Qaeda was seeking to extend its influence there on several fronts.

The use by charities of legitimately raised funds for military purposes was by then well established. In 1988, Mohammad Jafal Khalifa, bin Laden's brother-in-law, flew from Jeddah to Manila, the Philippines' capital, to establish a branch of the Saudi-based International Islamic Relief Organisation (IIRO).

Khalifa has since returned to Saudi Arabia and publicly criticised bin Laden's activities. However, Philippines' intelligence officers say the IIRO office and an array of associated business ventures have been used to channel funds to the separatist Abu Sayyaf Islamic group.

Since September 11, such global links have continued to be unearthed. In early November US authorities included al-Taqwa, a Swiss-based financial services company, on its third list of accounts to be frozen because of suspected links to terrorism. Ghaleb Himmat, the company's 63-year-old Tunisian-born executive and a board member of the Geneva section of the Kuwait-based International Islamic Charitable Organisation (IICO), was also on the list.

Himmat denies any connection with Islamic militancy. However a German intelligence report says the Munich-based Islamic Community of Germany whose president is Himmat expressed "pleasure" over the September 11 attacks at a closed meeting.

Much of the military experience of the current generation of fighters who have joined al-Qaeda was gained in Bosnia, where Muslim forces and communities have received millions of dollars of funding from Iran, Saudi Arabia and other countries.

Though most of the 4,000 foreign Muslims who fought as volunteers have now either left or settled down, "several tens have probably been involved in terrorist-related activities", said Tomislav Limov, Bosnia's deputy interior minister.

"During, and immediately after the war it was very easy to receive documents and remain here," he said "Many Afghan Arabs then married and had children. They considered themselves safe here."

Saudis pumped more than $600m into Muslim efforts in Bosnia, $380m of it from individual donations, the rest from the government. The Saudi government, which stripped bin Laden of his nationality in 1994, is aware that some of its wealthiest citizens today donate an estimated $1.6m a day to a variety of Islamic causes.

When it came to funding for the September 11 attack, money appeared to be no object, particularly in the months leading up to the hijackings.

The group used SunTrust Visa cards to spend thousands on rental cars, airline tickets and cash advances. The expenses included $100 given to northern Virginia counterfeiters for fake identifications, and $4,500 spent by two of the hijackers for business class seats on United Airlines flight 174, the second to crash into the World Trade Center.

Investigators are still tracking the sources of much of the $500,000 used by the hijackers. When the ultimate source of those funds is established it may be possible to stop the financing of terror. The question is whether governments that are asked to take tough action will have the will to do so.