On 9/11, we all wondered how 19 men had lived for a year among us and still given their lives to carry out such acts. Now people wonder even more at how young native British men would give their lives to kill random fellow Britons.

But there is something different here that troubles me. Most suicide terrorists ostensibly use this tactic because there are targets you can only attack if you give your life. Suicide was an essential part of flying a plane into a building.

But you very plainly don’t need to kill yourself to set off a subway bomb. Even with increased vigilence, anybody could leave a backpack behind, rush out the doors just as they are about to close, and then blow up the bomb as the train enters the tunnel, without dying or even getting caught (except perhaps on camera.) There was almost no tactical need for these men to kill themselves. Yes, it makes it a little more certain to do it that way, but we hope that committed terrorists (especially with such clean pedigrees) are not so many in number that they can be wasted this way.

But they were wasted this way, and I think deliberately. I suspect they were chosen not as the most committed, but as the most unlikely, just for the shock value, the idea that your neighbour could be a suicide bomber. And strong shock value it is, because while you don’t have to die to bomb a train, there are a lot of targets where willingness to die is tactically necessary to carry it off, and it’s close to impossible to defend against such attacks.

Those who think careful ID checks and national ID cards will stop terrorists now must step back. These kids had clean ID. The security cameras have helped discover the story but of course could do nothing to prevent it.

I have contended (to much opposition) that terrorism and suicide bombing is a tool used against democracies that are accused of oppression. A recent book (Dying to Win by Robert Pape) now backs this up. The answers are not good.