The attacks on London, Part Two: Why the search for the London bombers is still far from over

They may have caught the men who carried bombs on to the Underground, but detectives know they must now move fast to trace those whose brains, money and ideology made the attacks happen.

Nine suspected "foot soldiers" in the London bombings are either dead or in custody, but the hundreds of police, forensic scientists and intelligence operatives involved in the biggest terror investigation Britain has ever seen cannot afford to ease up for a moment.

Not only could more terror cells be waiting to strike, possibly taking their cue from the failed bombings 10 days ago, but the bombers and would-be bombers of 7 and 21 July must have had considerable support. They needed money, shelter, access to sophisticated detonators and - assuming they made their own explosives - some training in how to assemble the bombs.

Deep in the bare basement of London's Paddington Green police station, where all main terror suspects are taken, interrogators will be seeking the answer to one question above all: is there a mastermind at work? It may be the key to determining the size and shape of the network behind the bombers.

Investigators speak in terms of concentric circles, with the men of violence at the centre being surrounded by rings of sympathisers, bankers and bomb-makers. Penetrating this support structure is especially urgent because police are working in the belief that the groups responsible for the two rounds of attacks were self-sufficient, the second triggered two weeks to the day the first had taken place. With Thursday marking another fortnight since the last attempt, no one can be sure whether a third cell, seemingly unconnected with the others, is now preparing an attack.

Only by disrupting the leadership and support networks sustaining the would-be suicide bombers can any hope of an early return to normality be achieved. "Al-Qa'ida does not act like some classic Graham Greene cell," Sir Ian Blair said a week after the 7 July bombings. "It has very loose affiliations, and we have loose > affiliations, and we have got to find the bankers, the chemists and the trainers - all the people who are assisting in this."

But what do police really know about the men and women sustaining the plots? Within each cell there is likely to have been one ringleader. It seems clear, for example, that Mohammed Siddique Khan was the senior figure in the 7 July cell. At 30 he was several years older than the other suicide bombers.

The parents of the two younger Leeds-based Muslims in the cell said their son had come under the influence of a mysterious "Mr K": there is no evidence that this was anyone other than Khan. The former teaching assistant was also identified by Mohammed Junaid Babar, a convicted al-Qa'ida member in the US. Khan also knew one of the two British Muslims from Derby who staged a suicide bombing in Tel Aviv.

But who gave Khan his orders for the 7 July attack, and did he also instruct the leader of the 21 July cell?

Forensic evidence will be crucial in establishing any link between the two sets of bombers, even if they were not aware of each others' existence. Sources have indicated similarities in the type of explosive used and the rucksacks in which they were carried, but there has been no word on whether the bombs used the same batch of explosive. One of the main priorities will be to find pieces of the detonators used in the suicide bombings. If they are identical to those employed two weeks later, the connection would be conclusively proved.

In the meantime, speculation about a possible mastermind has fastened on a number of figures. Most attention has been devoted to Haroon Rashid Aswat, a British-born militant arrested in Zambia after he was incorrectly reported to have been held in Pakistan. He is now said to be telling his captors that he was once a bodyguard to Osama bin Laden.

If true, no Briton would ever have risen higher in the ranks of al-Qa'ida than Aswat. The name of the 30-year-old militant from Dewsbury emerged within days of the synchronised suicide bombings that killed 52 people. He was said to have slipped into Britain a fortnight before the attacks, and to have flown out hours before the four struck. According to rumours, he was in mobile phone contact with least one and possibly all four of the bombers, having made as many as 20 calls.

The only problem is that the rumours remain just that, and official sources are now doing their best to play down his potential role. "Any suggestion that he is linked with the bombings is wrong," said one Whitehall source. "The supposed mobile phone calls are a red herring, [although] that is not to say he is not of interest more generally."

Security sources believe Aswat is linked with terrorism, but outside Britain. He has been on the run since 1999, when he allegedly tried to set up training camps for al-Qa'ida in the US, and his arrest in Zambia was prompted by the FBI. According to British diplomatic sources, he is more likely to be extradited to the US than Britain.

Three of the four suicide bombers had close links to Pakistan, but the suspects in the failed bombings were all from east Africa, the other region in which al-Qa'ida has influence. America first became aware of the network's power when it attacked the US embassies in Nairobi and Dar es-Salaam with simultaneous bombings in August 1998. And even after 9/11 provoked a worldwide assault on the network, it struck again in November 2002. A hotel used by Israeli tourists in Mombasa was bombed; almost at the same moment a hand-held missile was fired at an airliner carrying holidaymakers home to Israel, but it missed.

As they seek to break into the concentric circles around the bombers, investigators have many more avenues to explore. Among them is disturbing evidence that Britain's prisons have become recruiting ground for radical Islamic groups. Several convicted and suspected British terrorists are alleged to have been radicalised while in custody, including Muktar Said-Ibrahim, arrested last week, who served two and a half years in young offenders' institutions. Richard Reid, jailed in the US for trying to bring down an airliner with explosives in his shoe, was drawn into radical Islam while in prison.

"When offenders first arrive in prison, they are told by the old hands to say they are practicing Muslims because the halal food is better," said one ex-convict. "Some cons are looking for meaning in their life and are drawn into or revert back to Islam. Unfortunately some inmates spout a pretty radical form of Islam." He added: "Prison is a ideal place for indoctrination as you have a captive audience, alienation and violence all mixed together." A leaked briefing paper to the Prime Minister warned: "Often disaffected lone individuals unable to fit into their community will be attracted to university clubs based on ethnicity or religion, or be drawn to mosques or preaching groups in prison through disillusionment with their existence."

More than 6,000 prison inmates in Britain give Islam as their religion. There are 130 imams employed in the country's 138 prisons, a few of whom have been suspended after they were alleged to have made anti-Western comments. But it is usually other inmates rather than imams who seem to draw offenders into radical Islam. In May this year the independent anti-terrorism watchdog, Lord Carlile of Berriew, warned ministers of the influence of extremists at one prison.

Jonathan Aitken, the disgraced Tory former cabinet minister, says efforts were made to recruit him to Islam during his prison sentence. He saw the recruitment of men like Richard Reid, the "shoe bomber", as no exception: "Rather, it is the militant face of a much broader phenomenon: the Islamic mission that is a new and astonishing feature of British prison life."

Another target of the investigation is the bomb-makers or chemists. Even if there was no overall mastermind, there may have been a figure with some expert knowledge, probably stemming from a degree in chemistry or a related subject. Such a person could have slipped into Britain and out again for the specific purpose of assembling the bombs.

The explosives found by officers investigating the London bombings were home-made, with the ingredients readily available on the high street. But the construction of the devices would have been difficult and dangerous. Detonators are more difficult to obtain, and might have been bought abroad. Other criminals leave a fingerprint at the scene of their offence, but bomb-makers leave a "signature" through the types of devices they build. Israeli police have conducted extensive forensic examinations of every bombing in Israel so that they can gain a better understanding of the techniques used and the profile of the bomb-makers. In some cases, links have been made from bombs used to carry out atrocities in one country to bomb makers living in another.

Yahya Ayyash, nicknamed "the engineer", was a notorious bomb-maker for the Palestinian group Hamas who is understood to have killed and injured hundreds. He studied chemical engineering at Bir Zeit University in the West Bank in the mid-1980s. His role as a bomb-maker came to an ironic end in 1996, when he was blown up by a mobile phone given to him by a friend who later disappeared.

One trail followed by investigators into the 11 September 2001 attacks in New York and Washington - the financing of the operation - may prove less fruitful in the new era of al-Qa'ida. While the 9/11 plotters needed pilot training on Boeing 747s and large numbers of air tickets to conduct reconnaissance for their mission, today's attacks tend to use home-grown terrorists operating on their own turf.

But there remains the fact that criminal networks, such as drug cartels, make money from their crimes and terror gangs do not: some cash is needed to finance their bombing campaigns and training camps for recruits. Without money they cannot operate.

But while investigators examine the myriad web of connections among the London conspirators on the one hand and al-Qa'ida's top leadership on the other, they have yet to establish any direct link between the two groups. One set is hiding in Pakistani cities and along the country's border with Afghanistan; the others are on our streets and in our council estates. Finding the common denominator is the priority.

Additional reporting by Graham Moonie

Who is behind the bombers?

"We have go to find the bankers, the chemists and the trainers - all the people who are assisting in this" - Sir Ian Blair, the Metropolitan Police Commissioner, 14 July.

The Banker

His funds will have been vital in setting up the terror cells, provided for training at overseas extremist training camps. Likely to have channelled cash through an off-shore bank account or even through the guise of a legitimate charity. The sophisticated web of financial transactions will make it difficult for investigators to penetrate and to unmask him.

The Chemist

Although the bomb materials could have been obtained on the high street, the bomb-maker would have needed expert knowledge to carry out the dangerous task of putting the bombs together. May already have experience in helping set up bomb campaigns abroad and have been educated in legitimate engineering or related subjects.

The Trainer

It is striking that suspects in both the 7 and 21 July attacks were bonded by a common interest in sport. Members of both groups are alleged to have met in gyms, and physical activity is a useful cover for terrorist training. Camps on the Afghanistan and Pakistan border have, in the past, trained thousands of jihadists.

The Mastermind

There is huge scepticism among security officials that the suicide bombers had the experience or capability to plan and carry out their attacks alone. The suspect most often put forward is Haroon Rashid Aswat, a militant arrested in Zambia who is alleged to have called the 7 July bombers, however, MI5 sources strongly downplay his significance.