In an instant the image is seared on the collective consciousness in the way pictures of ash-strewn figures groping their way through the streets of Manhattan were after 9/11.
It's a scene that might have come out of Rwanda or the Congo. Yet the setting is John Wilson Street, south-east London, with the surreal touch of a red double-decker bus in the background.
The man, later identified by acquaintances as Michael Adebolajo, does not flee. Nor does his accomplice, still not identified by police. The pair, who have just mown down and butchered 25-year-old British soldier Lee Rigby, roam the street, talking to stunned bystanders, blood-stained weapons still in their hands.
This is not just a brutal slaying. It's street theatre of the most macabre kind. The primitivism of the act seems a core part of the message: you don't need to master bomb recipes on the internet to shake a Western city to its core; you get a similar effect by picking up a blade and hacking someone to death in broad daylight.
Adebolajo's next act is to reach for the widest audience he can.
He approaches a terrified witness and, according to Britain's ITV, tells him that, "It's cool, I just want to talk to you." He speaks direct to the cellphone camera.
"The only reason we have killed this man today is because Muslims are dying daily by British soldiers ... it's an eye for an eye ... By Allah, we swear by Almighty Allah, we will never stop fighting until you leave us alone."
The impromptu press conference ensures news of the crime arrives simultaneously with the attacker's political message, a message that goes viral and runs far ahead of the delayed reaction of politicians and the authorities. Welcome to terrorism in the age of reality TV. Your modern terrorist wants to tell all to Big Brother, commentate on his own actions, be his own spin doctor.
Dr Ramon Spaaij of La Trobe University sees it as a "performative act, more than just a means to an end, an act which carries a particular meaning - it's saying no one is safe, anyone walking his dog could be butchered".
His immediate response was to think of the 2004 slaying of Dutch film producer Theo van Gogh, shot and then stabbed by his attacker because of a film seen as anti-Islam. Two knives were reportedly left in the body, one pinning down a five-page note.
Dr Brooke Rogers, senior lecturer in risk and terror at King's College London, believes the London pair were adopting the sound-bite technique of politicians, while sociologist Professor Kevin McDonald, of Victoria University, sees the staged nature of the London slaying as a "kind of narcissism" in which the perpetrators "become aware of themselves as a kind of social reality through the violence they engage in. It's very clear that being filmed and being visible through violence is very important to them."
Adebolajo's emerging story bears some striking parallels with the younger of the Tsarnaev brothers, who unleased hell on the streets of Boston last month.
The Briton was born to educated Nigerian immigrants, seemingly well integrated at school but radicalised in his later teens. Like Dzhokhar Tsarnaev, former schoolmates of Adebolajo's are expressing shock that the person they knew and liked could have evolved into someone capable of inflicting a harrowing atrocity on a total stranger.
"It seems odd to say it now, after the events of yesterday, but I remember him as just a lovely, lovely guy," the Daily Mail quoted former classmate Stephen Cavalier saying .
British media were quick to trace links between the London pair and hate preacher Anjem Choudary and it seems the pair had come to the attention of security agencies.
But others view the slaying as evidence of the rising "lone wolf" phenomenon among followers of violent, jihadist-oriented Islamism - a term describing single actors or a small group of actors who operate independently of any overarching organisation.
International security expert Neil Fergus, who heads the Australian-based consultancy Intelligent Risks, says the success of Western governments in disrupting the more co-ordinated actions of al-Qaeda and its high-profile offshoots has spurred the terror network to urge followers to take up DIY terrorism.
"I think it's a deliberate strategy by al-Qaeda through its arm in the Arab Peninsula to promote it ... and to encourage people to act like this," he says.
"Security and intelligence agencies have had increased concern and awareness about lone actors, principally because of the publications and actions of al-Qaeda in the Arab Peninsula. It goes back to bin Laden getting out of Tora Bora and saying now the organisation has to have many heads.
"Central planning was never a successful long-term tactic for al-Qaeda; there were too many ways that the plans could be, and were, compromised."
Spaaij, who last year wrote a book on lone-wolf terrorism and is being funded by the US National Institute of Justice to research the phenomenon, agrees.
"Al-Qaeda has lost a fair bit of its leadership and the rest has been forced to flee or scatter," he says. "So as organised terrorism becomes more and more disrupted, you see more and more of these splinter groups or lone actors appear." Yet in some ways it's not new, he adds. "Back in the 19th century, quite a few Eastern European anarchists advocated a very similar strategy because anarchists by definition were against any form of hierarchy or organisation.
"Now it's come full circle."
Nor is radical Islam the only seed-bed from which this kind of terror springs. Anders Breivik, who massacred 69 fellow Norwegians in 2011, was the classic lone wolf, operating with no outside support.
Experts have dubbed the new breed of DIY actors "self-starter terrorists" who often self-radicalise, initially through the internet or online message boards.
They don't have to search far. Online, they can turn to the e-zine Inspire, produced by a group known as "Al-Qaeda on the Arabian Peninsula", which reportedly helped the Tsarnaev brothers concoct their lethal pressure-cooker bombs.
Founding editor Samir Khan urged "all of the brothers and sisters coming from the West to consider attacking America in its own backyard" before he was killed in a US drone strike in late 2011.
Then there is the Lone Mujahid Pocketbook, another AQAP manual for the self-starter.
"There's a lot of chatter," says Rogers. "Online they are constantly looking at different approaches, different methods, different ways of shocking the [Western] public. We have an information environment that is absolutely flooded with ideas, with suggestions."
She believes the online web of extreme Islamists, with its many different communities and different levels of fanaticism, is in some ways more powerful than al-Qaeda ever was. "The sheer volume of material that is available is unprecedented. Individuals, if they're interested, can access it and to an extent 'self-radicalise'. It's almost sensory overload."
In 2010, 21-year-old Roshonara Choudhry was jailed for stabbing East Ham MP Stephen Timms with a kitchen knife. She had been a promising student, but while studying in her bedroom she was drawn to YouTube videos made by people explaining their reasons for converting to Islam. Ultimately that led to online lectures from another infamous hate preacher, Anwar al-Awlaki.
"If you go on YouTube there's a lot of his videos there and if you do a search they just come up," she told police interrogators.
In 2008 Parviz Khan, 37, of Birmingham, was sentenced to 14 years' jail for his plot to kidnap and behead a British soldier. In covert recordings he was heard planning the killing - and planning to film it, to ensure that the message about terror hit home in Britain.
Asghar Bukhari, of the United Kingdom's Muslim Public Affairs Committee, told the BBC that radicalisation was increasing as young Muslims were enraged by what they saw over the internet, such as graphic images of the collateral victims of drone strikes in Pakistan.
"This hatred is growing within some sections of the Muslim community," he said, blaming Islamic organisations for not doing more to channel this anger into legal forms of protest and activism.
But Rogers believes there have been greater efforts by mosques and Muslim communities to share information and more openly co-operate with the authorities. Paradoxically, this has had the effect of driving fanatics into darker corners where they are harder to detect.
"It's almost a double-edged sword," she said. "[Hate preaching] is happening more in front rooms in private homes, in private meetings, and, of course, online - where it's much more difficult to spot it, to find it, to track it."
Haras Rafiq, of anti-extremism think tank Centri, challenges talk of "self-radicalised young people".
"We are still as a community refusing ... to really tackle the groups that are underpinning and driving the radicalisation process," he told the BBC. "They may well have acted themselves but the question is, are they part of a wider network? Definitely."
In Australia there has been no emergence of someone as extreme as the hate-preaching Choudary in the UK, under whose influence Adebolajo allegedly fell.
However, Sydney's Sheikh Feiz Mohammed has been a controversial leader who has sailed close to the wind.
In 2007 police seized DVDs of his so-called "Death Series" in which he called on Muslim parents to offer their children to defend Islam. More recently he has softened his rhetoric and is thought to be co-operating with police.
Experts here say there are other messages out of London for local authorities. "I think the agencies are probably all now bracing for copycat attacks" says Dr Rajat Ganguly, senior lecturer in security studies at Murdoch University.
"I think the lone-wolf terrorist is probably going to be the most dominant form of terrorism, at least in Western countries. And countries where you have significant numbers of minority populations are the ones probably squarely in the firing line of this."
He also thinks the ethnic background of the London attackers might prompt terrorist-profilers inside the intelligence agencies to challenge preconceived ideas about which communities threats might emerge from.
"These are not typical examples of a person of Middle Eastern origin that you would normally think of when you think of something like this," he says.
The danger of acts such as the London slaying is that they prompt calls for revenge, as has occurred already with the right-wing English Defence League, emboldening radicals within every community.
Nine months ago the head of ASIO, David Irvine, gave a rare speech in which he warned of the needle-in-a-haystack challenge of allaying the threat from freelancers in terror.
"That lone wolf, that person who can be recruited, indoctrinated and trained over the internet until the very last moment and that can happen in a very quick amount of time before a bomb goes off, or whatever - that is the issue that ... keeps both me and my international colleagues awake at night," he said.
He will be hoping that what came to the streets of London does not ultimately make its home here, too.