Suicide Bombers: Britain's intelligence and security agencies were having to come to terms last night with something they had feared but hoped they would never have to face - the presence of suicide bombers in Britain.
It is the first time, not only in Britain but western Europe, that bombers have been prepared to commit suicide and completed the act.
In Madrid last year the train bombs were set off by timers triggered by mobile phones. Some of the bombers were prepared to commit suicide but only when they were cornered later by the Spanish police.
Yesterday we learned that for the first time suicide bombers - four of them - had carried out an attack in Britain, choosing the most vulnerable of targets.
Furthermore, the bombers in the view of the security services were British born and bred.
Not only that: they could plot the attack without being detected, either by MI5 agents and informants or by the security and intelligence officials scanning e-mails and intercepting telephone calls looking for suspicious communications.
What concerns the security services is that the four bombers appear to have been "radicalised" in Britain, not indoctrinated in training camps and religious schools in the Middle East.
How young men apparently from stable backgrounds - as well as from broken or unstable families - are attracted to commit such atrocities has concerned MI5 and the Home Office for a long time.
A senior MI5 officer is understood to have addressed a meeting of G8 home and interior ministers on the issue in Sheffield last month.
Security sources said yesterday that ministers would have to look again at radical clerics who can encourage extremism and influence young men disillusioned with Western culture.
It seems clear that MI5, the domestic security service, needs to build up its network of agents, an anti-terrorist official said yesterday. It is already setting up regional offices in Britain. "Agents are essential," a senior official said last night.
He compared the task facing MI5 to looking at a blank piece of paper.
"The four bombers are in the middle. You then go out from there, look at their pasts, where they met, what they had done in the past, where they had travelled, who they associated with."
That should help the security and intelligence agencies to build up a picture, not only of these four bombers, but the extent of the potential threat posed by other suicide bombers in Britain.
In an interview with BBC London yesterday morning after the security services and the police had made their breakthrough, Sir Ian Blair, London's Metropolitan Police commissioner, said it was likely there would be another attack, although he insisted the terrorist threat could be defeated.