The fact that Westminster suspect Salih Khater was apparently unknown to counter-terrorism police and MI5 before crashing his car into barriers outside Parliament yesterday will, if an extremist motive is confirmed, unsettle security officials.
In most recent attacks it has emerged soon afterwards that those involved were known radicals already subject to scrutiny by our spies and police. That, of course, often leads to criticism about “missed opportunities”.
But the security agencies are, perhaps paradoxically, largely reassured when they have previous knowledge of an attacker because it means their intelligence-gathering methods are working well — even if decisions on which “subjects of interest” to prioritise remain difficult to get right every time.
By contrast, yesterday’s incident — yet to be officially declared as a terrorist attack after inconclusive searches of Khater’s home overnight — could turn out to be another example of the problem of “clean skin” attackers striking after only a short period of radicalisation.
Such cases are far harder to prevent because there is little opportunity for security agencies to spot signs of a potential attack through online activity, contacts with other extremists or purchases of potential weapons.
New legislation, to strengthen penalties for “propaganda” offences which lead to radicalisation, is passing through Parliament.
But no complete solution exists, and with 676 active terror investigations, there appears little prospect of the threat from unknown attackers diminishing soon.