It's a grisly crime that -- if carried out -- would have shocked Australia to its core. And it was plotted by a schoolboy on the other side of the world.
Britain's youngest terror mastermind has been jailed for life for orchestrating the beheading, which was to have been carried out during a parade in Melbourne on Anzac Day -- a national holiday honoring the country's war dead -- in April this year.
The boy, who admitted directing the jihadist plot and encouraging others to take part, was just 14 when he planned the brutal slaying. He cannot be identified because he is a minor.
Detective Chief Superintendent Tony Mole, head of England's North West Counter Terrorism Unit, said the boy's role was "quite shocking," considering he was "extremely young."
"I think it shows that the ideology, if you're open to it, it takes no prisoners ... there are certain people who fall into the seductive propaganda of some of the ISIL stuff that's pumped out on social media.
"He's been caught up in that, he's explored it and he's escalated into an attack plan, and a credible one, which is an extremely dangerous thing to do."
According to authorities, the attack was to have been carried out in person by his co-conspirator, Sevdet Besim, from Melbourne; the pair had set out the details of the deadly outrage in thousands of messages sent using an encrypted app.
But the plan was foiled, police say, when they were called in after the teenager threatened to behead teachers at his school in northern England, prompting counterterrorism experts to crack the encryption code on his smartphone. Besim awaits trial in Australia on charges of conspiring to commit a terrorist act; he has not yet entered a plea.
Alerted by their counterparts in the UK, Australian police closed in and found "the knife, the flag, and the martyrdom ... script," Mole said.
The boy, who is now 15, was known to have behavioural problems, but his parents, who are divorced, are said to have had no idea their son had been radicalized until police became involved.
The British-born teenager had managed to convince 18-year-old Besim that he was much older and had a history of radicalism, testing the Australian's religious knowledge and determination to carry out an attack.
"He's put himself in the space of authority and Besim has accepted that," said Mole. "That's the mask of social media -- you can, if you [behave] in the right way, ask the right questions, you can show yourself to be that sort of mature person that Besim was ... looking for to give him some guidance."
Authorities claim that over the course of nine days the pair exchanged some 3,000 messages using controversial messaging app Telegram.
Security analysts say members of terror groups like ISIS use encryption apps including Telegram, Surespot, Kik and Wickr to send messages to each other without the risk of them being read by outsiders.
"It is very well known that ISIS -- and not just ISIS -- uses open source social media like Facebook and Twitter to circulate its propaganda," said Charlie Winter, of counter-extremism think tank Quilliam.
"What you also see is people in Syria and Iraq who self-advertise as Islamic State fighters and recruiters and they provide the details to their Surespot account, their Kik account, their Telegram account."
British authorities want the ability to monitor such communications.
"Do we want to allow a means of communication between people which -- even in extremis, with a signed warrant from the Home Secretary personally -- we cannot read?" British Prime Minister David Cameron asked in January this year.
His message has been echoed by Andrew Parker, the head of Britain's security service, MI5, who says counterterrorism forces must be able to monitor suspected extremists.
"MI5 and others need to be able to navigate the internet, to find terrorist communication," Parker told the BBC in early September. "We've been pretty successful in recent years but it is becoming more difficult as technology changes faster and faster.
"We need to be able to do in the modern age what we have done in our history -- [we] need to be able to monitor the communications of terrorists and spies."
Telegram's co-founder, Pavel Durov, says he is "sorry" that the teenager was using his app to plot the beheading, but insists: "If Telegram did not exist, this young boy would have used some other app."
A Russian exile, Durov says he's seen first-hand what happens when a government has too much power over information.
"What I saw in countries like Russia [and] many other places is that when law enforcement bodies get access or can get access to the data eventually it leads to abusing that kind of power."
He says people have the right to secure communication, and warns it is almost impossible to limit the spread of encrypted technology.
But later this year, the British government plans to introduce draft legislation dealing with encryption -- in the hopes of stopping the next teenage terrorist.