At around 9.15 on the morning of October 26, just six weeks after terrorists struck America, an apparently ordinary young man in casual clothes boarded a Boeing 747 at Manchester Airport.

As the Pakistan International Airlines jet banked over the city's outskirts and the rain-sodden Cheshire countryside, the young man settled down for a journey longer than even he could have imagined. For his final destination would be a wire cage in the baking heat of Camp X-Ray, the US military base in Cuba where suspected al-Qaeda terrorists are held.

Acquaintances in the blue-collar Midlands town of Tipton knew the 24-year-old as "Shaf". His tastes, they say, were indistinguishable from millions of other young British males. He liked football, girls and nightclubs.

But the next time Shafiq Rasul's whereabouts can accurately be pinned down - December 13 - he was in Shibergan prison near Afghanistan's border with Turkmenistan. He was wearing traditional Afghan dress and had been arrested by the Northern Alliance on suspicion of being a foreign volunteer fighting for the Taliban and Al-Qaeda.

Mr Rasul, and two friends from Tipton, Ruhal Ahmed and Asif Iqbal, "did not realise what was going on, or how much trouble they were in", according to a Kabul-based official with good contacts at Shibergan. Instead "they had a relaxed attitude" - apparently thinking they would soon be returning to Britain. Indeed, the burning question Mr Rasul had for western officials was how well his beloved Liverpool was doing in the Premier League.

Today, following the transfer of the trio to Cuba, Mr Rasul and his friends face trial by a US military tribunal able to recommend the death sentence.

The process by which "Shaf" became one of the terrorism suspects described as the "worst of the worst" by the US military remains unclear. He had told family members he was travelling to Karachi to take a course in the Windows XP operating system more cheaply than was possible in the UK.

Whether Mr Rasul is eventually found to have been a dangerous terrorist or just an idealistic foot soldier of a cause he believed was just, he represents the elusive target in the war against terror - moving undetected from Britain's crowded inner cities and run-down suburbs via a loose network of al-Qaeda sympathisers, through the check-ins and security controls of the world's airports to the battlegrounds in the holy war waged across the globe.

This is the story of the young men who set out on their missions after September 11, when every security service in the world had been placed on red alert. Some, like Mr Rasul, were caught. Others were killed. But intelligence operatives and analysts everywhere are saying there are hundreds, perhaps thousands more they just cannot see.

One such intelligence official is Frank Spicka who has spent the past 17 years as a bodyguard to American presidents. But in all his life in the line of fire he was never as worried as he is today.

"Even if [Osama] bin Laden was killed, this is an organisation that can carry on. The head might be cut off, but the body is already too extensive to die with it."

The catastrophic failure of intelligence which let 19 hijackers slip through the net and change the world on September 11 catapulted him to the heart of the global war on terrorism as the first US secret service agent to be seconded to Interpol as head of the force's terrorism department.

Mr Spicka's appointment was a mark of how vital transatlantic co-operation and intelligence sharing has become in the global strategy.

But far from assuring themselves of victory over terrorism, European and US security officials admit that not only has the trail for Osama bin Ladin and his associates gone cold, but that there remains a huge gap in the detailed knowledge of the current dimension of al-Qaeda.

Since the declaration of war on terrorism and the vast military, police and intelligence operation it has entailed, al-Qaeda or its affiliates have been thwarted in several terrorist operations, albeit at the last minute in some cases, according to US officials. However, most of the organisation's leadership is at large, they have won new recruits, and and they appear still to have money, friends and safe houses to support them.

"There is growing evidence that many individuals were evacuated from Afghanistan before the major military offensive. It's safe to assume that many of them have entrenched themselves elsewhere in the world," Mr Spicka said. "But what we have learnt of al-Qaeda's global reach is that it is a mistake to see this simply as a movement of Arabs and Afghans."

Even the most conservative estimate circulating within European security circles is that there could be up to 4,000 terrorists at large prepared to follow the mission of bin Ladin. President Bush last month put the figure at 100,000. As one French intelligence officer said: "No one is feeling comfortable that we're on top of it."

Referring to prisoners held in Afghanistan and at Camp X-Ray, one security analyst put it bluntly: "They've caught quite a few foot soldiers, most of whom have no contact with the decision making of al-Qaeda. It's debatable just how much the US-led offensive has contributed to our detailed knowledge of al_Qaeda as it is today."

British anti-terrorist police are now operating on the assumption there could be a hard-core of up to 100 supporters of bin Ladin in the UK, any one of whom could turn to terrorism.

Just as the first wave of Islamic militancy to hit the Middle East in the early 1990s was the result of trained fighters drifting back home from Afghanistan after fighting the Soviet occupation forces, there are now growing concerns of a similar, though much wider ripple effect as al-Qaeda fighters flee Afghanistan while the global network of cells which could support their activities is still in place.

As a western intelligence officer said: "Trying to get to grips with the threat is going to be a long painstaking process and its not going to come quickly. If the public is looking for weekly round-ups they are going to be disappointed."

Mosques, universities, and charities, where anti-US feeling is widespread, are under continuous monitoring but the hard evidence that can turn intelligence into a successful prosecution remains elusive.

While militant groups in Britain which had been extremely vocal in their criticism of the US military action in Afghanistan have now gone largely quiet, Muslim community leaders and the security services regard them as having gone underground rather than disappeared.

Two groups that have been banned from university campuses in the UK are still present. Al-Muhajiroun now organises most of its talks to university students under different names. Literature sourced to the other group, Hizb ut-Tahrir, is freely available in Islamic prayer rooms at a number of universities.

One article found on display last month was produced by the group a week after the September 11 attacks and was headlined: "Alliance with America is a great crime forbidden by Islam."

The literature confirms these groups' stated rejection of terrorism, but says: "As for Jihad to fight the enemy who commits aggression against Muslims, usurps their land, plunders their resources and attempts to control them; not only is this a legitimate matter but it is an obligation. It is the highest peak of Islam."

This literature has been in circulation among Muslim students using the prayer room at the University of Central England, in Birmingham. Their number formerly included Mr Rasul.

Added to this threat of trained al-Qaeda fighters who may have escaped from the Afghanistan conflict, is a new generation of recruits among disaffected Muslim youth.

"We are not talking about the threat just of the old mujahideen veterans of Chechnya, the Balkans, and Afghanistan but also of a youth that is susceptible to propaganda fuelling itself on events since September 11," one senior Spanish security official said.

This new reality was evident in the anxiety of a Muslim community leader whom the FT agreed simply to call Mohammed when he agreed to be interviewed secretly on a housing estate in the British city of Leicester. Mohammed's concerns reflected the dangers facing the Muslim mainstream if it speaks out against militancy while seeking to divert the youth from extremism.

Mohammed said "a handful" of young men had disappeared from the city since September 11 and had made their way to Afghanistan. Nobody knew how they arranged their journey. Today, nobody knows whether they are dead, captured or on the run.

Mohammed was prepared to discuss the existence of Afghanistan fighters from Leicester because he wanted non-Muslims to understand the peril he believes the UK faces as a result of the war on terror. The Muslims' cause was increasingly being misunderstood, he said.

"People will not give up this cause. It does not help the situation in the UK that the Americans have broadened their hostility to Muslims by accusing Iran of promoting terrorism. There will be scope for more recruitment in the future. But at present everyone is lying low." The world's intelligence services, governments and police forces have been forced to refocus on such communities. It is only in the aftermath of September 11, that British security and police officials have acknowledged there could be dozens of sleepers and potential recruits within the Muslim community in London and the Midlands.

"Many of the arrests since September 11 were the result of old work on people monitored over years. But there is now a network being reconstructed in Europe ... more secretive, more radicalised, and with a greater capacity for violence," said Guillaume Dasquie, a French anti-terrorist expert and editor of Intelligence Online.

The young men of Leicester are part of a growing list of British Muslims discovered to be associated with al-Qaeda or to have fought alongside the Taliban.

The first step on their path to Afghanistan, said the community leader, was their exposure to the most extreme elements within radical Islamic political groups and movements that have been targeting UK mosques and universities since the early 1990s.

The organisations Hizb ut-Tahrir and its offshoot, al-Muhajiroun claim to have several thousand British followers. Together they comprise the Khilafah Movement, whic h aims to establish a universal Islamic state.

The British security services have long dismissed the groups as any real security threat. This view continued even after September 11. But the emergence of British citizens in al-Qaeda's ranks has transformed the picture, while wrongful arrests in the UK have only served to whip-up radicalism among some Muslims.

"The mistake by the Home Office was to treat these guys as at worst a joke and at best a nuisance," says Manzoor Moghal, who sits on the national executive of the Muslim Council of Britain. "Now they've woken up."

Despite the non-violent ideology of al-Muhajiroun and the now secretive Hizb ut-Tahrir, Mohammed says the "fringes" of the groups provide a space where more zealous supporters can actively seek out other individuals.

This was the second stage for the young men of Leicester.

"You can find them easily through existing groups," he says, even if it is just a mobile phone number.

The process of being guided down a chain of activists through a series of mobile phone calls need not even beclandestine. Up and running again after being taken off the web is www.qoqaz.com. The site was used by the alleged "20th hijacker" Zacarius Moussaoui, now on trial in the US, to encourage Muslims to seek military training and take up arms in Chechnya.

Meanwhile, Al-Muhajiroun's Pakistan-based website provides the mobile phone number 0092 300 8414956. This belongs to Hassan Butt, a 21-year old British Muslim born in Luton who is now based in Islamabad. Until he diverged last month from al-Muhajiroun's official line that its members protest peacefully against the war in Afghanistan, Mr Butt was its spokesman in Pakistan.

He remains an active member of the group, though increasingly appears to be following his own path, notably by advising Muslims on how to gain military training.

"Whenever I send someone over [to Afghanistan] it's not me as al-Muhajiroun. It's 'you trust me and I trust you'. I've no problem with helping anybody who wants to come to Pakistan seeking military training," he told the FT.

In the organisation's premises on Tottenham High Road in north London, Sheikh Omar Bakri Muhammad, the group's founding leader, says of Mr Butt: "As a Muslim I believe he is there practising his divine priorities. But as al-Muhajiroun we've focused on removing the system."

Asked whether al-Muhajiroun's Pakistan connection could make it easier for these individuals to be recruited to armed groups, he replies: "There are offices of all jihadi groups functioning openly," Sheikh Omar said. "So it is not difficult."

Hassan Butt's frankness has earned him a reputation for hyperbole - a charge he denies: "It's just a matter of time before operations are taken out in Britain. When it does happen people will bite their bottom lips," he said.

Many months spent canvassing students in the UK made him well known among those sympathetic to al-Muhajiroun's cause.

One of the areas he canvassed was Tipton, home of Shafiq Rasul and his two friends Asif Iqbal and Ruhal Ahmed. Mr Iqbal is also being held at Camp X-Ray. Mr Ahmed is being held at Kandahar and a fourth Tipton man, Munir Ali, is missing.

Mr Butt insists that he did not recruit the Tipton four, but admits to visiting the town, and describes himself as a "signpost or guide" for would-be jihadis rather than a recruiter.

"I've aided many people to go into Afghanistan for many reasons ranging from humanitarian work to military struggle. But my main work is political work against the illegitimate government of Pakistan. We are working politically towards an Islamic government in Pakistan which will then facilitate a real jihad - with nuclear weapons."

For the time being, radical Islam's volunteers seem more likely to take their cue from the strategy of the September 11 hijackers. One apparently would-be suicide bomber is another Briton, Richard Reid, the so-called "shoe bomber" who was overpowered by fellow passengers

on a Miami-bound American Airlines flight from Paris after a flight attendant saw him trying to use a match to ignite explosives hidden in his shoe.

Investigators on both sides of the Atlantic have been piecing together the details of the Reid case. Some say it points to the emergence of a new generation of converts to the cause of terrorism, even less predictable than those who flew passenger aircraft into the twin towers and the Pentagon, though no less zealous. They form part of a pool of recruits and a support network still capable of wrong-footing the biggest international counter-terrorist and security operation mounted in modern times.

Reid's apparent lack of sophistication as a terrorist has been pointed out by security experts who have analysed the bomb he tried to use. But, say others, there was a netw ork that was prepared to use him: "They knew there was a volunteer prepared to do what they wanted him to do."

Security officials are growing in the belief that Reid may personify a new generation of terrorists. Picked up from the gutter of society after a spell in a UK prison in the mid-1990s - where he converted to Islam - he went for training at al-Qaeda's Khaldan camp in Afghanistan in 1998, was hardened by the US offensive in Afghanistan in the aftermath of September 11, and was prepared to die for the cause.

But as the war on terror moves more into its post-Afghanistan phase, security services are now faced with the task of not only identifying such individual suspects but also the painstaking process of understanding the real character of al-Qaeda. Building up a profile has brought an expansion in the fields of investigation, way beyond the "natural" well of support for the organisation among Muslim militants.

For instance, there is strong evidence to support claims that Osama bin Laden and his associates have built up a dedicated Al-Qaeda fleet of small freighters, operating on the fringes of the shipping industry.

"The whole shipping business is based on false identities. Shipowners head the list using shell companies but false identities and qualifications are part of the mix too," says David Cockroft, secretary general of the International Transport Federation.

It is in this netherworld that al-Qaeda's fleet of vessels is thought to operate. Industry insiders believe between 10 and 80 vessels could be under the control of al-Qaeda or associates.

But it is not the vessels under direct control of Al-Qaeda that authorities consider to pose the greatest risk to international security. There is concern that an organisation capable of the suicide hijackings of airliners could just as readily turn to major shipping targets.

And then there is the question of al-Qaeda's reach, which is more one of loose connections than one of organisational sructure.

"Before you ask, 'where has al-Qaeda gone now?', you must ask first, 'what is truly al-Qaeda?'," said a Pakistani businessman with a history of business dealings with Islamic groups. "If you believe there was a central core and then lots of smaller cores tied to the centre but capable of operating independently, you begin to understand how this group operated."

There is evidence of such cores across south and south-east Asia. India recently made its first four arrests of individuals it insists are al-Qaeda members in Bombay and Hyderabad, and claims to have "conclusive" evidence against one of them, a top security official told the Financial Times.

Elsewhere, in what would have been the most devastating terrorist attack since September 11, activists trained and advised by al-Qaeda almost succeeded in detonating seven truck bombs - each containing three tonnes of ammonium nitrate explosive - at targets across Singapore, that included western embassies, office buildings and military facilities.

The plot was foiled in December with the arrest of 13 members of the Jemaah Islamiah, a region-wide group present in Singapore, Malaysia, Indonesia and the Philippines. The group, whose existence in Singapore only became known last September, has liaised closely with al-Qaeda.

Lee Hsien Loong, Singapore's deputy prime minister, told the FT: "They had reached the point where their handlers said: you get the stuff (ammonium nitrate), another group will come and rig the bombs, and a third group will come and mount the operation."

The Singapore plot depended upon a network which straddled the region, one that is still operational as its key members are still in hiding - it is widely believed - in Indonesia's Riau islands.

The plot revealed the logistical capacity of al-Qaeda affiliates, despite the US-led "war on terrorism" being conducted throughout the period during which the finishing touches were being put in place.

Most threatening is the tendency of the al-Qaeda network and its affiliates in south-east Asia to exploit the political instability in Indonesia. In December, Lt.Gen. Hendropriyono, Indonesia's intelligence chief, said al-Qaeda had set up training camps on the island of Sulawesi.

Intelligence sources have tracked the flow of weapons from the southern Phillippines to Sulawesi. They have also uncovered a 15-page document written in Arabic called "Operation Jihad in Asia". The document, a copy of which was obtained by the FT, details plans for attacks planned for last December 4 on US embassies in Malaysia, Singapore and Jakarta.

The scale of the plot has sent a shudder through the region, and has contributed to the realisation among intelligence services that they still have no definitive picture of what to expect from al-Qaeda and the groups with which it is associated.

"One of biggest fears is the threat from unidentified sleepers placed prior to September 11," said a western intelligence source. "If you consider that the attacks of September 11 were years in the planning and months in their preparation, it is quite possible that another terrorist operation could be in the pipeline - and the endgame could be as early as tomorrow."

Reporting by: Jimmy Burns in London, Paris and Madrid; Jonathon Guthrie and Gautam Malkani in London, Leicester and Tipton; Farhan Bokhari in Islamabad, Victor Mallet in Paris, Mark Odell in London, Andrea Felsted in London, David Stern in Baku, Edna Fernandes in New Delhi, John Burton in Singapore, Tom McCawley in Jakarta and Yogjakarta, Taufan Hidayat in Central Java.