With some terrorists there are few, if any, warning signs, little intelligence and thus virtually no opportunity for the authorities to do anything to prevent an attack. Some on the security services have sought to make the case that Jamal al-Harith was one such individual, or as close enough to it to make adequate surveillance of him disproportionate to the threat he posed – which was deemed minimal.

The fact that he had been held unlawfully at Guantanamo, had been released and never faced any charges, let alone gained a conviction for any crime, is the fundamental reason why he obtained his official compensation. That provides some explanation, if not justification, for the failure to monitor his activities.

Why would the state snoop on someone who has, officially, done nothing wrong? How could the authorities have stopped him in any case if he wasn’t in custody? How are they to know he was undergoing radicalisation?

Those points have been well made. It is also fair to say that Guantanamo’s excesses were responsible for driving many to the further reaches of extremism, such as the former Ronald Fiddler. Thus, Harith plainly did pose a threat to life, when he blew himself up near Mosul. At some point, we now know, Harith was going to commit such an action – and it could have just as easily been in Britain.

Hindsight is not issued as standard equipment to operatives in the British security services; but, according to David Blunkett, the regime of automatically putting those released form Guantanamo under surveillance was reversed after the Conservative-Liberal Democrat Coalition took power in 2010. Harith’s murderous mission, Lord Blunkett is strongly implying, is something that would not have happened on New Labour’s watch. Even discounting for a degree of self-serving partisanship, there are indeed questions to be asked of the person who was Home Secretary at the time these policies were being revised – one Theresa May.

Assyrians struggle in Lebanon after fleeing Isis jihadists

Even if such surveillance wasn’t practical, or had Harith evaded it in some way in any event (security is never perfect), there is the fresh allegation that the authorities failed to block the terrorist path to Syria that Harith, and no doubt others, took a few years ago. This represents a much more serious claim of incompetence, and poses some further difficult questions about the diligence of the security services.

Once again they have not been able, or chosen not, to put their side of the story publicly. Perhaps it would be unwise for them to do so. We also know that they succeeded in thwarting many terror attacks in this country and abroad. Even so, if they could have delayed or prevented Harith's trip to the Middle East they would have saved many lives, even though he might have chosen instead to commit his act of jihad on UK soil.