As investigators piece together the background to the Westminster terror attack, the picture beginning to emerge appears to bear the hallmarks of a “lone wolf” strike that has long concerned security services.
Since the July 7 bombings in 2005, counter-terror agencies have improved their capacity to uncover major plots involving several individuals.
Cells made up of a number of members are more vulnerable to surveillance of their communications, and any length of time spent planning a large-scale attack raises the risk of appearing on the authorities’ radar.
But the danger posed by lone individuals who adopt low-tech methods such as using knives or vehicles to strike suddenly is far less predictable.
The prospect of single assailants, and the possibility of them being radicalised over the internet, has been repeatedly highlighted by senior security figures.
No group has been officially linked in connection with Wednesday’s attack in which three people and the attacker died, with police saying only that the perpetrator was inspired by “international terrorism”.
But suspicions are likely to turn to Islamic State. British Defence Secretary Michael Fallon has said the working assumption is that the Westminster attack is linked to Islamic terrorism.
In December a report from EU law enforcement agency Europol said Britain was among the countries which are “high on the target list” for aggression from the group.
And the paper said the scale and impact of “lone actor” attacks is increasing. It added: “The majority of attacks claimed by IS appear to be masterminded and perpetrated by individuals inspired by IS, rather than those who work with the organisation directly.”
Following recent attacks in Europe, focus has fallen on a message delivered by IS spokesman Abu Muhammad al-Adnani more than two years ago. In a speech released in September 2014, al-Adnani urged followers to rise up against Westerners using any means available.
He said: “Smash his head with a rock, or slaughter him with a knife, or run him over with your car, or throw him from a high place, or choke him or poison him.”