The attack was unprecedented. A British soldier run down and beheaded in a London street in the middle of the afternoon.
Britain has spent millions of pounds since the 2005 London Underground attacks to prepare against the threat of terrorism, yet here, in broad daylight, two men brandishing a meat cleaver and a butcher's knife, have shown how cheaply that veneer of security can be punctured. The barbarous horror of freelance jihad has reached the high street.
The nation is in shock, but this has been security chiefs' greatest fear, an attack straight from the playbook of the new, ''lone-wolf'' brand of terrorism.
In 2008, Parviz Khan, 37, of Birmingham, was sentenced to 14 years' jail for plotting to kidnap and behead a British soldier. In covert recordings he was heard making plans for the killing - and to film it, to ensure the terror message hit home in Britain. ''This was not only a plot to kill a soldier but a plot to undermine the morale of the British Army and inhibit recruitment,'' High Court judge Justice Richard Henriques said.
It is not known if Wednesday's attack was inspired by Khan's failure. Clearly, some elements have been ''improved'': why bother to film it? Just carry it out in broad daylight, and let the smartphone-carrying public do the rest.
Newsrooms in London tied themselves in knots: they could not ignore the story but they were well aware that by putting a bloody-handed alleged terrorist on their front page or news bulletin, and by quoting his message in screaming headlines, they were doing exactly what he hoped for.
This was an attack from the al-Qaeda handbook. The terrorist group's English-language magazine, Inspire, produced in the Arabian Peninsula, exhorts people to carry out ''lone wolf'' attacks. Articles and advertisements urged readers to use knives and run people down in vehicles, the Daily Mail reported.
The first issue of the magazine contained an article ''How to build a bomb in the kitchen of your mom'', which was reportedly used by the Boston Marathon bombers.
This switch to a ''self-starting'' philosophy has not been overlooked by counter-terrorism agencies.
Last year, then MI5 director-general Sir Jonathan Evans said: ''I suspect that any problems we may have here will come from lone actors attracted to extremism and violence. In back rooms and in cars and on the streets of this country there is no shortage of individuals talking about wanting to mount terrorist attacks here.''
But one of the main problems for security forces is separating dangerous ''individuals of interest'' from noisy political agitators.
Little of al-Qaeda or its communications network remains. In its place are radicalised individuals, families or small groups, armed with ideology, informed by the internet, and free to interpret these ''instructions'' however they wish.
Since the July 7 London bombings in 2005, security forces have used electronic surveillance and border security to identify jihadists who travelled overseas for training. Only half of MI5's work involved plots with a Pakistan or Afghanistan dimension, said Sir Jonathan, with Yemen and Somalia the new priority areas.
But much of the ''education'' and radicalisation now happens on home soil, or online. YouTube and other sites carry video sermons from radical clerics.
One of the most-cited is Yemen preacher Anwar al-Awlaki - who was killed by a drone strike in 2011.
The co-founder of Inspire advocated what some officials dubbed the ''just do it'' brand of terrorism, writing, ''Assassinations, bombings and acts of arson are all legitimate forms of revenge against a system that relishes the sacrilege of Islam.''
Awlaki's sermons are continually put on YouTube and other sites, as officials work to have them removed. In one video he advised, ''Don't consult with anybody in killing the Americans - fighting the devil doesn't require consultation.''
Roshonara Choudhry, 21, who was jailed for stabbing East Ham MP Stephen Timms with a kitchen knife in 2010, told police she had been radicalised by Awlaki ''explaining stories from the Koran and explaining about jihad''. ''If you go on YouTube there's a lot of his videos there,'' she told police.
A lot of the radical material online mentions beheading, quoting a verse from the Koran on ''smiting the necks'' of unbelievers.
British authorities have met to determine whether they need to brace for more attacks. But even if the latest murder was not part of a broader plot, it is unlikely these will be the last lone wolves to attempt acts of terror on British soil.