Despite all the security, despite the intensive activity of the police, the intelligence services and the authorities in the past few days, terrorists have struck again in London. Exactly two weeks after the underground and bus bombs that killed 56 people and left 700 injured, bombers have gone for the exact same targets — the transport system. It almost beggars belief. It was bad enough that terrorists managed to strike in the first place. That they could do so again 14 days later, even though they did not manage to kill anyone, is no less horrifying. Londoners are understandably in shock. When it wears off, they will be angry as well — not just at those responsible, but at the fact that a fresh attack could have been allowed to happen. The frightening reality is that yesterday’s blasts could have been as devastating as the earlier ones. That they were not is no thanks to the efforts of the British police and or the authorities. It is thanks to the fact that those who carried them out did not have sufficient explosives. They had only detonators. The intention to kill was there; what was lacking was the means.

The use of detonators alone suggests that this was a copycat attack carried out by inexperienced fanatics rather than a follow-up mastermind by the same terrorists responsible for the earlier one. Inexperienced they may have been, but they were no less motivated — and the bombings were clearly coordinated as well. That is extremely worrying — because unless security on the London transport system is tightened up, there will be others. Other extremists will copy the copycats. The tactic works — and there are other extremists out there, not just Islamic ones.

Saying that this latest attack was no thanks to the efforts of the British security services is not a criticism of them; they have managed to discover a great deal about those involved in the earlier attacks in a remarkably short amount of time. It is a statement of the extraordinary difficulty they face in preventing terrorists attacks on a transport system as open and as free flowing as London’s. As elsewhere in the world, the transport system is the city’s Achilles’ heal. Londoners are a resilient lot. Bombs are nothing new to them. They had to contend with World War II, with the IRA, and now with the militants. They will carry on regardless. Bombs only stiffen their resolve. They are certainly not going to stop using the transport system. But given that other militants will inevitably go for the same targets, it is no longer an option saying that it impossible to police the London underground, that to tighten up security would bring it to a halt.

It has gone beyond the days of asking passengers to be vigilant or watch out for unattended bags. That is no good when people are prepared to blow themselves up. Better security systems have to be developed. It may mean armed police in disguise on every train; it may mean screening. It is time to think the unthinkable.