This was the nightmare the authorities had feared most - suicide bombings in Britain 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.
The first suicide bombings in Britain show just how much the country is on the front line.
Until now the images of militants blowing themselves up belonged to television pictures from the Middle East, Iraq and Chechnya. The security agencies will have to find how this happened in Britain and work out how to prevent more.
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 "homegrown" terrorists who travelled to the city on the east coast main line to kill and maim fellow residents and to die in the process.
Along with recriminations about the failure of intelligence over the London bombings has been speculation they were 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.
Databases, names of suspects, and information from allied services in Europe and the Middle East are available.
What they are faced with instead appears to have been a small cell of Britons, hitherto unknown to the authorities, who simply got on trains and a bus with their deadly packages.
Such acts are unlikely to be affected by Chancellor of the Exchequer Gordon Brown's pronouncements on tracking and seizing international terrorist funds.
The whole London operation probably cost less than 1000 ($2600).
The painstaking forensic tests have given the police a fairly accurate idea of the bombs used and they are trying to find out whether the bombers had contact with foreign groups to smuggle in explosives.
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. Home Secretary Charles Clarke has admitted the bombings came out of the blue.
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 surveillance footage at Kings Cross station.
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 security.
Lord John Stevens, the former Scotland Yard commissioner, 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 3000 British Muslims have passed through paramilitary training camps in Afghanistan and Pakistan.
The vast majority, it was thought, had divorced themselves from extremist activities after returning to this country.
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 Spanish investigation 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 the Spanish police managed to hunt down the terrorists. The British authorities can only hope they have similar success.
* 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 do suicide missions?
* Just how many so-called "homegrown 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 Britain?
* Why did it take the authorities so long to admit publicly that suicide bombers may have been involved?