Beyond any deluded claim to be serving Islam, the killer of Muslim French soldiers and Jewish children in southern France had something strikingly in common with the Charlie Hebdo murderers, the assassin at the Jewish museum in Brussels and the Moroccan accused of trying to cause a bloodbath on the Amsterdam-Paris express. Each was known as a potential threat to security.
To the names of Mohamed Merah, the Kouachi brothers, Mehdi Nemmouche and now the train attacker Ayoub El Khazzani can be added those of suspected terrorists behind a string of atrocities in Europe.
Time and again, the earliest reports identifying assailants are followed by revelations about the extent to which their suspicious conduct had already reached the eyes or ears of intelligence services. But let us look beyond knee jerk assumptions that these dangerous men are allowed by official incompetence to slip through the net.
There are many hundreds of people, probably a few thousand across Europe, whose activities might arouse suspicion. They may have travelled without good cause to conflict zones or become recognised as advocates of extremist violence. Yet security specialists are correct when arguing that keeping each of them under surveillance would require extraordinarily labour-intensive operations.
That does not necessarily mean we can do nothing but hope, as in the case of El Khazzani, that brave individuals will foil attacks.
Few politicians have felt bold enough to mention the possibility that internment, in other words the detention of suspects without trial, could be a useful weapon in the battle to keep the public safe or at least safer.
Those opposing such a move understandably point to failures of the British policy of interning paramilitary suspects in Northern Ireland in the 1970s.
Yet there is reason to believe a carefully constructed programme of internment could operate fairly and effectively in the exceptional circumstances in which we find ourselves.
Restricting immediate thoughts to EU countries, I see an absolute need for a policy of detaining suspects to be a shared responsibility, costs included, of the entire bloc. If only France, say, or Britain or Belgium introduced internment, it would almost certainly backfire.
There would have to be conscientious action to achieve justice. Judges or special panels, or both, could be empowered to overrule each decision on the detention of an individual. And while the greatest current threat is unquestionably posed by disaffected people who happen to be Muslims, the interests of equality demand much wider application. There are neo-Nazis, whether antisemitic or just anti anyone different, Islamophobic thugs and others, including even the more militant of animal rights activists, whose activities could rightly provoke a similar response if strong suspicion exists but there is insufficient evidence for prosecution.
No one should fear summary imprisonment because their beliefs may seem obnoxious. The peril comes not from what people think, but from what they do or may do.
Internment is a blunt instrument. It is not difficult to see it acting as a recruiting sergeant, though in an existing climate of radicalisation with society already under ferocious attack, this danger ought not be exaggerated. Desperate problems sometimes require desperate responses and it is surely within reason to commend the proposal as one meriting rational discussion.
If internment stifled one plot to slaughter the innocent, it would justify itself.
Colin Randall is a former executive editor of The National