But a review of court records and statements by officials suggests that the violence in London and Tehran was more than just a message. It reflected persistent efforts by the Islamic State since its rise in 2014 to hit targets once thought unassailable — especially in Britain. During this period, officials there intercepted and foiled more than a dozen plots, including five in the past three months.
The number of disrupted plots appears to be far greater in Iran, a Shiite-majority country loathed by the militant Sunni extremists of the Islamic State, which has aimed to hit Iran since at least 2007. A day after the deadly assault last week on the Parliament building and the tomb of Iran’s revolutionary founder, Iranian intelligence officials said they had thwarted 100 terrorist plots in the past two years.
Hours after the violence in Iran, the Islamic State released its glossy, online magazine, directly challenging skeptics who have questioned the group’s stamina as its territory shrinks. “What many of these analysts failed to admit, however, is that losing territory was nothing new for the Islamic State,” the article said, referring to the group’s near-defeat in 2011 at the hands of United States forces in Iraq.
“The reality faced by the Crusaders today is that despite their claims that the Islamic State has been weakened,” it said, “strikes in the heart of the Crusaders’ strongholds in the West will continue to occur just as suddenly and unexpectedly.”
While few details have been shared with the public about thwarted attacks in Iran, plots neutralized in Britain show how the Islamic State’s reach grew with each attempt. The techniques used by the frustrated attackers, the types of targets they chose and the kind of coaching they received broadly follow the arc of the group’s evolution as it struck repeatedly elsewhere in Europe.
The earliest instances involved would-be attackers who had ideological affinity for the Islamic State but no direct communication with the group. They took their direction from the many YouTube videos they had watched showing Islamic State atrocities, and from detailed instructions delivered online by Abu Muhammad al-Adnani, the group’s spokesman.
The messages initially called recruits to travel to the group’s territory, but as those journeys became more perilous, Mr. Adnani exhorted adherents to stay put and wage violence at home.
That shift is seen in a note riddled with typos scrawled by an 18-year-old suspect in Peckham arrested with a backpack concealing a hammer and knife: “Because I have no means ov gettin there I will wage war against the british government on this soil,” the suspect, Brusthom Ziamani, wrote in June 2014, according to a definitive report by the expert Raffaello Pantucci on the growing terrorism threat. “The british government will have a taste ov there own medicine they will be humiliated this is ISIB Islamic States of Ireland and Britain.”
Initially seen as the acts of disaffected youth with no real connection to the Islamic State, these plots did not at first cause alarm, officials say. But in the space of a year, they began to morph into something more complex, as young perpetrators began making online contact with the militant group.
“The rules of engagement changed,” said Max Hill, a British barrister who has prosecuted several high-profile terrorism cases and is now the country’s independent reviewer of antiterrorism legislation.
“We moved from a traditional understanding that there needs to be a trainer — a radicalizer — who is physically present into an era where there may be some remote encouragement coupled with fast access to radical material,” Mr. Hill said. “Those factors alone are enough.”
Just months after the Islamic State declared its territory-holding caliphate in 2014, the authorities began to see suspects who were in contact with British nationals who had traveled to join the militants.
Such was the case for three men who lived near Southall, a suburb of London, and who maintained contact with a friend who had slipped across the Syrian border, according to court records.
While their friend joined the ranks of the Islamic State, the three remained stuck in Britain, where they increasingly steeped themselves in the group’s propaganda. In September 2014, when the militants’ spokesman, Mr. Adnani, broadcast an audio recording inciting followers to kill in any way possible — “smash the disbeliever’s head with a rock or slaughter him with a knife, or run him over with your car” — the young men felt as if he was talking to them, the records show.
The three began obsessively shopping for knives, discussing different models in chats punctuated by the sharing of graphic videos. “How was it? … Like butter?” wrote one of the three, Haseeb Hamayoon, after posting a clip of the beheading of a British aid worker, according to a transcript of the message. “Any moaning? Niceeeee.”
The men were arrested in November 2014, after two of them were seen walking into a Kitchen Ideas store, where they bought a Victorinox knife and a sharpener, according to documents from their trial.
It is unclear if their contact inside the Islamic State played a role in their progression toward violence, or if he merely acted as a point of inspiration. But by 2015, investigation records reveal, plotters began tapping the inner core of a network of Islamic State online coaches. These coaches spent their days sitting behind computer screens in Raqqa, the Syrian city that is the group’s de facto capital, moving the impressionable men they met on the internet like chess pieces.
They were part of an organized unit nicknamed “The Legion” by the F.B.I., which began tracking them and picking them off in American drone strikes when it became apparent how involved they had become in remotely inciting attacks, including in the United States, Canada, Australia and Britain. One of its best-known members was Junaid Hussain, a British citizen who was among the Islamic State operatives believed to have been in touch with one of the men involved in the first successful ISIS attack in America, messaging the shooter as he drove to a community center in Garland, Tex., in May 2015, where he would open fire.
In the summer of 2015, Mr. Hussain used the encrypted app Surespot to reach a 24-year-old named Junead Khan in Luton who had fallen under the spell of the Islamic State. Mr. Hussain’s stated purpose for reaching out to Mr. Khan was to inform him that a friend of his who had joined the militants had been killed in Syria in an American drone strike, said Mr. Khan’s lawyer, Andrew Hall.
Mr. Hall believes that was a ruse. “The purpose of the call was to use it to call for revenge,” he said. “By coming to Syria and taking his place. Or, alternatively, by doing an action in England.”
Mr. Hussain offered Mr. Khan addresses of British soldiers and a nine-page guide to building a backpack bomb. Court transcripts also show that the young man was obsessed with knives.
“He had a longstanding fascination with a particular sort of knife,” Mr. Hall said. “He kept looking at it on Amazon.” Naming an Islamic State executioner who decapitated Western hostages, the lawyer said, “It was the sort of knife rumored to be used by Jihadi John.”
Counterterrorism experts say the number of foiled Islamic State plots represents the tip of the iceberg. At least 500 people are under active surveillance in Britain, they say, raising the question of whether all can be stopped.
“Everyone is saying enough is enough, but what can you actually do?” said David Wells, who until 2013 worked for the Government Communications Headquarters, Britain’s version of the National Security Agency.
“My view is that there is some tinkering around the edges that you could do,” he said, “But the foreign fighter community being as large it is in Europe, it’s a 20- or 30-year problem — and no one wants to hear that right now.”
The authorities suspected Mr. Khan, along with an uncle, Shazib Khan, had been plotting to attack the R.A.F. Lakenheath air base, home of United States Air Force 48th Fighter Wing, according to Mr. Pantucci, the terrorism expert. He tracked the evolution of Islamic State plots in Britain in a report for West Point’s Combating Terrorism Center. But court records show Mr. Khan had been planning not just to crush people with a van but also to stab them, following the instructions outlined in Islamic State publications.
Several months before they were arrested, Shazib Khan recorded a video of a car driving past the House of Commons in Westminster. He uploaded it to YouTube under the title “ISIS drives around Westminster.”
The footage was set to a chant addressed to the leader of the Islamic State, Abu Bakr al-Baghdadi, and included the lyrics, “The virgins are calling, record me for my martyrdom.”
The location filmed was a five-minute walk from where another Briton aligned with the Islamic State, Khalid Masood, killed four people in March at Westminster Bridge. That was the Islamic State’s first successful attack in Britain.
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