The great fear lurking at the back of the public consciousness after the London bombings of two weeks ago was that this would not prove a unique outrage, but the beginning of a campaign of terror. Were these four murderers working alone, people asked themselves? Or were they part of a wider network of bombers, primed to descend on the capital? Yesterday, while London was still mourning those who died on 7 July, a series of co-ordinated explosions on the city's transport system seemed to confirm our worst fears. Terrorists had struck again.
At the moment, we do not know if the four small explosions were part of a follow-up to the atrocity. The simultaneous timing of the attacks was certainly in the al-Qa'ida style. And the fact that three Tube trains and a bus were targeted is a chilling echo of two weeks ago. But this does not mean the same hand lies behind yesterday's explosions. There are suggestions that it might have been a copy-cat attack. The small amount of damage done and the failure of the explosives to take any lives suggest that this may be the work of a group less well trained in the horrible techniques of mass murder. This is far from comforting, of course. But it is a relief that London is not today mourning the loss of more innocent lives.
Yet whatever the nature of the group that perpetrated yesterday's attacks, it makes the debate about security in the capital more urgent. It is clear that London is now a major target for terrorism - and that the city's transport system makes it especially vulnerable. There is also an unmistakable symbolic element to these recent attacks. It is suspected that the desire of the 7 July bombers was to create a "burning cross" across London. And yesterday's attacks were also spread across the centre of the capital. All this, sadly, means it is sensible to assume that this will not be the last time London is attacked in this way.
The Metropolitan Police responded well to yesterday's crisis. In such situations the authorities find themselves in an invidious position. There is intense pressure on them to release details. But giving out erroneous information could prove disastrous. Yesterday the police - rightly - stuck to the rule not to release information until being sure of the facts.
There is now a strong case for greater measures to protect the Underground system. Airport-style security for those taking bags on to the Tube should be put in place. The London Mayor, Ken Livingstone, was asked at a press conference yesterday what the effects of heightening security at Tube stations in this manner would be. He estimated that it would result in a 15- to 30-minute increase in journey times. That sounds inconvenient, but not unreasonable.
Some will dismiss this out of hand. But the public tends to adapt fairly well to security measures when it can see the point of them. This would be the case with more rigorous checks on the Underground. Implementing this in outlying stations would create a technical challenge, but it is not insurmountable.
It is always important to bear in mind that London cannot be made wholly secure. That is true of all great, open cities. And as Stanley Baldwin remarked in a different context: "The bomber will always get through." Ultimately, the best thing individual Londoners can do, as the Metropolitan Police Commissioner, Sir Ian Blair, urged last night, is carry on as normal. The battle against those who would cause chaos and murder on the streets of our capital city will be difficult and long. But with the aid of smart security measures and better intelligence, it can be won.