Our prisons are saturated with risk. Prison staff managing terrorist offenders are on the razor-sharp end of this spectrum. National security does not stop at the prison gates. As we have seen, high walls and physical incapacity do not of themselves cure minds blighted by hate.

In 2016 I visited HMP Whitemoor in Cambridgeshire and sat down with frontline officers. The most striking thing was the matter-of-fact way they described the hazards of being in close proximity to unrepentant killers. Their biggest fear was of being taken hostage and murdered, probably by beheading, in a filmed act of terrorism.

I was put in mind of those sombre exchanges when one of the threats I predicted would happen in 2016 came to pass as two