Could it really happen again?

The first arrests in connection with the London bombings have been overshadowed by gaps in intelligence and the fear that a third cell may be planning to strike

Many things have changed dramatically in Britain over the past four weeks. It would have seemed inconceivable before 7 July and the deaths of 52 innocent people that the London transport system might ever be flooded by 6,000 police, many carrying semi-automatic weapons, let alone that the sight would be welcomed by most commuters. But in the wake of two attacks on London, one unsuccessful, and a breathless wait on Thursday to see whether the passage of another fortnight would bring a third, the paramilitary presence on buses and the Underground brought nothing but reassurance.

The four bombers of 7 July are dead and those alleged to have tried to replicate their murderous assault two weeks later are in custody. Six people have been charged under terror laws with failing to disclose information about them, and several more are still being interrogated at the high-security Paddington Green police station in London. But the Government, the security services and the public need an urgent answer to the question: is there more to come?

A desperate hunt for a third cell of bombers is reported to be under way. According to The New York Times, a recently intercepted mobile phone text message, and the interrogation of a terrorism suspect being held outside Britain, have given rise to fears that another team of terrorists might be planning synchronised attacks in London, or elsewhere in the country. It said the information came from senior counterterrorism and intelligence officials based in Britain, mainland Europe and the US.

Meanwhile, intelligence agencies are warning the Government that Britain is facing a potential insurgency rather than a sporadic campaign of terrorist acts. Their assessment is based on the fact that the country is harbouring tens of thousands of young men from the Horn of Africa, Afghanistan and its neighbours who can handle an automatic weapon. An estimated 10 per cent have basic training in light weapons and military explosives. Even though the vast majority have come to Britain to escape the lawlessness of their homelands, there remains an alarmingly large pool of people who are open to radicalisation and already have the capacity for armed violence.

Predictably enough, al-Qa'ida did its best to exploit this climate of fear. As it has done in the wake of other attacks, such as the bombings in Madrid, the movement sought to give the London atrocity its retrospective blessing. Ayman al-Zawahiri, the Egyptian physician who serves as Osama bin Laden's right-hand man and chief ideologue, proclaimed in a videotape that the London bombs were Tony Blair's fault for deploying troops in Iraq. "God willing", he added, there would be more destruction.

Holed up somewhere between Pakistan and Afghanistan, Zawahiri and Bin Laden almost certainly knew nothing in advance of the 7 and 21 July attacks. Nor is it likely that they are aware of what might be coming next. But that is not the point: as security experts have been pointing out for some time, al-Qa'ida is now an ideology and an inspiration rather than an organisation directing acts of terrorism, although that does not make it any less dangerous, in the view of George Kassimeris, a terrorism expert at the University of Wolverhampton.

"In the kind of terrorism we are experiencing now, ideological training is more important than logistical or operational training," said Dr Kassimeris. "For al-Qa'ida, propaganda and symbolism are massive assets, which is why Zawahiri spoke exactly four weeks after the 7 July bombings. He wanted to aggravate British fear and insecurity."

If al-Qa'ida were a coherent network, it might actually make the task of the police and security services slightly easier as they seek to head off further attacks. Instead, the little that has emerged from the investigations into 7 and 21 July has simply made the threat seem more amorphous and confusing.

Far from requiring large amounts of money, training and co-ordination, the attacks appear to have been largely spontaneous and self-generated.

Hussain Osman, suspected of the attempted bombing of a train at Shepherd's Bush, is reported to have told his interrogators in Rome, where he was arrested on the strength of a British tip-off, that the conspirators were angered by Iraq rather than fired by extreme Islamist ideology. Instead of praying, they gathered in the basement of a gym, where they discussed the news and their jobs, and watched videos of the fighting in Iraq.

An Italian police official, Carlo de Stefano, said Osman had no links to known terrorist cells. He added that investigations "lead us to believe as very probable that he belongs to a spontaneous group rather than a structured organisation that had broader terrorist projects". The suspect, who used to live in Italy, was described by a former girlfriend as her "handsome Hamdi-Bambi", who loved rap music, America and girls.

The history of the 7 July bombers, said Dr Kassimeris, emphasised what he called "the ordinariness of terrorism". He added: "It is the 'boy next door' syndrome. These people are not superhuman, but nor are they brainless losers, as some have sought to depict them."

This, however, makes them harder to detect. "We have learnt that a very small group of determined people can inflict a great amount of damage, but until last month we have been looking in the wrong places and following the wrong targets. We are not dealing with groups of full-time terrorists, but people who engage in terrorism almost as a kind of hobby.

"And terrorists do copy one another. In an earlier era the Red Brigades in Italy copied the Red Army Faction in Germany, which copied Action Directe in France, and the same thing is happening now. They learn from each others' mistakes."

Thanks to American officials who have passed on to the US media what they have been told by their British counterparts, we know that considerable advances have been made in some areas of the investigation into the July bombings.

The 7 July bombs were initially thought to have been made from military explosive, then from TATP, a peroxide-based material which can be made from easily obtainable ingredients. But New York detectives told a public briefing that both the 7 and 21 July bombs were made from HMTD, another volatile home-brewed explosive which can be concocted from hair bleach, food preservatives and the kind of tablets used in self-heating rations.

To the horror of London investigators, the New York police officials went on to reveal that the 7 July bombs were detonated by the alarm setting on mobile phones, while those a fortnight later were rigged to detonate by hand.

It also emerged that the suicide bombers had bought industrial refrigerators to store the explosive they made in a shabby Leeds flat, and used cool boxes to transport their bombs in two cars as far as Luton, from where they took a train to London. Their operation appears considerably more professional than the attempted bombings two weeks later, where less careful handling of the explosive may have made it deteriorate to the point of uselessness.

But while the police can claim a great deal of success, both in capturing suspects and in the forensic investigation, huge gaps remain, mainly on the intelligence front. "It is depressing that we have not managed to scoop up more of these people," one security expert said. "My sense is that we haven't got very far in finding out who they are. We haven't penetrated many groups."

Until the 21 July attempt, investigators believed that the 7 July bombings and a string of failed terrorist plots in the past could all be linked to a network. So far, however, it has been impossible to establish whether the two groups of London bombers were connected or aware of each others' existence. For every clue that indicates some degree of collusion, such as the common choice of HMTD as the explosive, there are others that point in the opposite direction, for example the different methods of detonation. And for all the talk of a "mastermind", there is very little so far to connect the bombers directly to al-Qa'ida. Mohammed Junaid Babar, a convicted member of the network, is reported to have told US interrogators that he recognised Mohammed Sidique Khan, the presumed leader of the 7 July bombers, but it is not clear where that has led. "I suspect there were three or four people in technical roles who haven't been caught because they slipped away before the attacks," the security expert said.

Nor have tantalising clues to overseas involvement yet produced a breakthrough. All three of the 7 July bombers who grew up in Leeds are understood to have spent time in Pakistan, Khan and Shahzad Tanweer travelling there together. But despite reports that Tanweer met British-born terror suspects being held in Pakistan, no evidence has emerged that the bombers received training there. If anything, it appears that they were radicalised in the land of their birth rather than that of their parents.

"People don't need to go to Pakistan to be trained," said the security expert. "There is so much knowledge and capacity in this country already that it's scary."

Mr de Stefano said Hussain Osman had made a call on his mobile phone to one registered in Saudi Arabia as he travelled by train between London and Rome, but nothing else has emerged about any connection outside Britain to the 21 July suspects. On Friday, however, it was disclosed that the four suspects had all used multiple aliases and addresses. Not only are they suspected of claiming up to £500,000 in benefits under different names, but two are alleged to have used forged passports to claim asylum.

The next major development in the investigation may be the return to Britain of Haroon Rashid Aswat, the Yorkshire-born al-Qa'ida suspect being held in Zambia. One of the 7 July suicide bombers is said to have had his telephone number, but British security sources insist that he is of far more interest to the Americans, who claim he was involved in alleged attempts to set up a terror training camp in Oregon. Zambia, however, is insisting on deporting him to the land of his birth rather than the US. It will be interesting to see what Britain does with him.

British authorities may be making progress with the origins and methods of the London suicide bombers and suspects, but little of this will help to prevent the next attack. The security agencies fear they are facing the enemy they always dreaded: home-grown, autonomous groups with the desire and capacity to savage the society they live in, the society of which they appear to be everyday members.