This was the nightmare scenario that the authorities feared most - suicide bombings carried out by British citizens leading seemingly ordinary lives, slipping under the radar of the security agencies.
What emerged yesterday transforms the investigation into the London attacks. These, the first suicide bombings in western Europe, put into grim context just how much Britain is now on the front line. Until now the images of militants blowing themselves up had only been seen in television pictures from Palestine and Israel, Iraq and Chechnya. The security agencies will now have to unravel how this was replicated in Britain and work out how to prevent a recurrence.
The task they face is daunting. At the end of a dramatic day of raids and arrests, a few stark facts have emerged - the men who bombed London were "home-grown" terrorists, who travelled to London to kill and maim fellow residents of this country, and to die in the process.
Along with recriminations about the failure of intelligence over the London bombings, over the past four days there had been speculation they had been the work of foreign insurgents.
Scotland Yard helped to add to this impression by urgently requesting information from European security agencies about north African suspects and dismissing reports of suicide bombers being involved for as long as possible to avoid panicking the public. Intelligence sources acknowledge that dealing with an attack by foreign Islamists would have been easier. There were available databases, recognisable suspects, and tranches of information from allied services in Europe and the Middle East.
What they are faced with instead appears to have been a small cell of Britons, hitherto unknown to the authorities, who carried out a fairly unsophisticated operation by simply getting on to trains and a bus with their deadly packages.
Such acts are unlikely to be affected by pronouncements from the Chancellor, Gordon Brown, on tracking and seizing international terrorist funds. The whole London operation probably cost less than £1,000.
The painstaking forensic tests have given the police a fairly accurate idea of the type of bombs used and inquiries are under way as to whether the bombers or their associates had been in contact with foreign groups to smuggle in explosives and manufacture the device.
But Robert Emerson, a security analyst, pointed out: "These appear to be pretty simple devices, easy to put together from manuals, or the internet, pretty cheaply. What we saw last Thursday was a pretty base-level operation. The critical advantage the terrorists had was that they were unknown. There are certain to be other, similar, groups out there. Then we have the biggest problem, what sanction can you have against someone who is already prepared to give up his life."
Lack of intelligence remains the biggest problem. Charles Clarke, the Home Secretary, has admitted that the bombings "came of the blue" and the police and MI5 had been caught completely unaware.
Yesterday's development showed the great pace at which the investigation has moved. The main reasons for this, however, have been the discovery of items belonging to the bombers at the site of the bombings and CCTV footage at King's Cross station. It was these leads which allowed the security agencies to sift through the information which came in huge amount from the public and make their move yesterday.
It is this bank of information and the inroads made by yesterday's raids and arrests which will form the basis of the investigation from now on.
The police and the security agencies will be able to establish the contacts of the bombers, the mosques they attended and trace a wider circle of sympathisers.
But removing a few heads of the hydra does not guarantee future security. John Stevens, the former commissioner of the Metropolitan Police, disclosed at the weekend that during his tenure there were eight separate extremely serious plots by "home-grown terrorists" - and each one involved a different group.
According to MI5, about 3,000 British Muslims have passed through paramilitary training camps in Afghanistan and Pakistan. The majority, it was thought, had divorced themselves from extremist activities after returning to the UK. Senior police now believe, however, that there is a far greater pool of recruits for a British insurgency, fuelled by anger over the Iraq invasion, than previously thought.
Much had been made of the similarities between the London and Madrid bombings. The commuter trains in the Spanish capital were not blown up in suicide attacks, but, like here, took place simultaneously at the height of the morning rush hour without warning.
At the end of a 14-month investigation, Jorge Dezcallar, the former head of CNI, the Spanish security service, said: "This was a local sleeper cell. It may have been inspired by al-Qa'ida, but it had no links with Osama bin Laden. Some of the bombers were thieves and petty criminals. They did not even have an Islamic past. They are almost impossible to detect."
The Spanish investigation, too, benefited from luck. A van containing Islamic tapes and traces of explosives was found at a station car park. Then a bag retrieved on a train, initially thought to belong to one of the victims, yielded an unexploded bomb. From such breaks Spanish police managed to hunt down the terrorists and plug vital information into the international security system. The British authorities can only hope they have similar success.
The unanswered questions
* How did this group remain undetected to make their attack?
* Were they working with associates who are preparing further bombings?
* Did the group receive bomb-making material from abroad? If so, where?
* If this was an autonomous cell, how many more are there?
* And do they also contain people prepared to carry out suicide missions?
* Just how many so-called "home-grown terrorists" are active at present?
* Have these people been involved in armed struggles abroad - perhaps in Iraq, Palestine, Afghanistan - and returned to carry on the war in this country?
* Why did it take the authorities so long to admit publicly that suicide bombers may have been involved?Reuse content