The terror attack in Westminster on Tuesday fits an increasingly familiar pattern.
An attacker, using a car to mow down pedestrians and a knife to assault police, went on a deadly rampage at the heart of one of Europe's great cities, seizing headlines and putting terrorism centre-screen once more.
By late Tuesday afternoon, no claim of responsibility for the attack had been made. But social media and messaging channels used by the Islamic State in Iraq and Syria (ISIS) were predictably awash with crowing celebrations of the deed soon after news of it broke.
It was a year to the day since ISIS' attack in Brussels, in which three coordinated suicide bombings killed 32 and wounded hundreds more. ISIS has shown a particular fondness for anniversaries in the past.
In the months since Brussels, and the European security clampdown that has followed, the terrorists have also exhorted and guided their followers to undertake exactly this kind of assault.
The Westminster incident recalls those claimed by the group in Berlin last December and Nice last June. In those circumstances, lone attackers, without active support networks to easily arm or train them, were manipulated and radicalised rapidly in communications with ISIS to act. They used trucks to ram into crowded gatherings of civilians.
Such attacks may be the pattern for the future.
In London, the response from the security forces was swift. The parliamentary estate is ordinarily heavily policed: the attack lasted moments before police gunfire stopped it. Scotland Yard's restructured counter-terrorism response forces were put to their first real-life test. Armed teams were on the scene quickly.
In the new model of digitally mediated terror, and with a greater number of radical persons of interest to keep tabs on than at any time before, no place can claim to be completely immune.
Reassurance from such a response can only go so far, however. A marauding assault - such as that in Paris, with multiple attackers on a less-heavily defended site, or a location in a crowded area less easily reached by police forces, such as London's West End - may have been far harder to contain.
Urban counter-measures, such as concrete bollards around sensitive buildings or public areas to prevent vehicle attacks, have limits too. Public thoroughfares, such as Westminster Bridge, by their nature cannot be made invulnerable.
As ISIS comes under more and more territorial pressure in its heartlands in Syria and Iraq, its incentives to strike out against the civilian populations of its adversaries will meanwhile only increase.
Spectacular bomb and gun attacks will probably remain the terrorists' goal, but opportunism will probably be the more dominant factor, say Western security officials. Simpler attacks, as those in Nice, Berlin and now London show, need not be less deadly.
Details of ISIS' attack model that have become clearer to law enforcement agencies investigating the spate of atrocities in Europe and the US in recent months bear out such expectations.
ISIS has a multi-layered approach to planning attacks abroad, say Western intelligence officials. It calls upon both trained operatives - returnee fighters from its ranks in Syria and Iraq - and radical or vulnerable individuals who may never have actually joined the network or met others in it.
The authority to organise attacks is devolved to key lieutenants in its attack planning department. Many are Europeans. They run networks of direct, often skilled and trained acquaintances in Europe who aim to facilitate Paris-style attacks.
They may dispatch individuals to directly perpetrate actions, as appeared to be the case with the Sousse beach massacre. They also seek to recruit and link digitally with individuals or networks of radicals - people they may not directly know or may never have met in real life - guiding and training them online.
It is like a game of chess, says one European security official: "Some are just pawns. Some are more powerful - small groups, maybe - and some have even more flexibility and resources."
Until now, Britain has taken succour from its natural advantages in the West's fight against ISIS. The country's borders and tight gun laws make larger-scale, coordinated attacks in Britain harder to pull off. Security forces also have a long and bloody history of handling them.
But in the new model of digitally mediated terror, and with a greater number of radical persons of interest to keep tabs on than at any time before, no place can claim to be completely immune.
"There will be terrorist attacks in this country," the chief of MI5, Britain's domestic security service, warned in a November interview last year. "The threat level is severe and that means likely."
•The writer is the Financial Times' defence and security editor.
A version of this article appeared in the print edition of The Straits Times on March 24, 2017, with the headline 'Westminster attack fits increasingly familiar pattern'. Print Edition | Subscribe
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