Since the atrocities of July 7, 2005, Britain has had to be on full alert in the very real knowledge that an attack by international terrorists could be mounted at any time. When arrests such as those that occurred just before Easter take place, the public holds its breath. Was this the real thing? Or was it the authorities overreacting?

The release without charge of 10 of the 12 suspects will cause public opinion to shift more to the idea that those in power are scaremongering. The view that an attack is a very real possibility will lose credibility.

The difficulty with investigating the threat posed by al-Qaeda, as I found it, is knowing when to make your move. It is one thing to eavesdrop on telephone calls or e-mails. It is a completely different discipline to turn those conversations, which are not admissible in court, into evidence that passes the jury test.

The huge responsibility that goes with the job of fighting terror is judging the balance between when to intervene to stop a possible attack and when to wait, to allow events to unfold so that incriminating evidence can be collected. If you wait too long, there is the danger that public safety is threatened; but waiting that extra hour could be the key to getting the golden nugget that secures a conviction. It is a test of nerve.

The secret is to be in control. Keep the suspects under intensive surveillance and, by doing so, minimise the risk of attack. If the suspects move, you are alongside them. In this case, it appears that the authorities moved to the arrest phase because they thought that an attack was imminent. That was at the expense of waiting a little bit longer to collect evidence, or to establish the lack of it.

Andy Hayman is former Assistant Commissioner for Specialist Operations in the Metropolitan Police

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