News dominated all day by revelation that man shot dead by police was nothing to do with the bombings.

This is the ultimate nightmare for the police. Cool heads are what we need. And no rush to judgement – as knife edge decisions that may save us citizens from being blown up are clearly difficult for the security forces to make.

This tragedy should not have happened. We are used to the idea that if someone is down – there shouldn’t be a need to shoot them in the head. But elsewhere it’s much more common – how else can you be sure to stop a suicide bomber? We would not want it the other way round – that they didn’t kill through hesitation a bomber – giving him long enough to detonate a deadly device. And the person detonating does not need to be the same person carrying the bomb either.

Tough choices. Clearly this appalling error needs to be investigated so that police (if it was the police) can reduce the potential for such a mistake in future. But let us not fool ourselves – in the circumstances we face at present – these tough choices won’t go away.