It may help explain why the Islamic State had not, as of Wednesday night, asserted responsibility for the deadly rampage the day before in Manhattan by a truck driver who was wounded and arrested by the police, who say he is a disciple of the group.
With few exceptions, the Islamic State has not claimed attacks when a surviving recruit falls into the hands of the authorities.
“It’s as if he didn’t even exist,” said Jean-Charles Brisard, director of the Center for the Analysis of Terrorism in Paris, regarding the omission of the 10th attacker from the group’s propaganda.
The Islamic State has followed this practice regardless of whether the attacker was directly dispatched or inspired remotely through online tutorials.
Among the attacks that investigators have definitively tied to the group — and which it never claimed, despite the casualties they caused — were those at the Jewish Museum in Brussels in 2014, which left four dead, and on the Thalys train in 2015, which wounded three.
Last year, when Ahmad Khan Rahimi, who planted bombs in New Jersey and in Manhattan’s Chelsea neighborhood, was finally taken into custody, police also found his bloodstained journal, containing references to Abu Muhammad al-Adnani, the Islamic State spokesman and the leading figure calling for attacks on the West. But ISIS did not take responsibility for Mr. Rahimi’s acts.
More recently, the Islamic State never claimed a truck-ramming in Stockholm in April, even though the perpetrator was found to have been a recruiter for the group.
Nor did the group take responsibility for a vehicular attack in Edmonton, Canada, just over a month ago, carried out by a man who took the time to drape the Islamic State flag on the dashboard of his Chevrolet Malibu before using it to ram through a traffic barricade.
The constant in each of these cases is that the perpetrator was apprehended.
More than 24 hours after the arrest of the driver in the Manhattan assault, which left eight people dead, the group’s media arm remained mum, even though he had left a note stating that the “Islamic State will endure forever,” a reference to the group’s slogan.
The lack of a claim is conspicuous in light of how much territory the group has lost in Syria and Iraq in recent months, erasing 90 percent of the caliphate it once held. Claiming responsibility for the truck rampage in New York, the worst terrorist attack in the city since Sept. 11, 2001, might be expected to project an image of strength for the group.
A New York Times tally of the more than four dozen attacks on Western targets claimed by the group since 2014 indicates that the Islamic State typically issues its claim of responsibility within 24 hours, though there have been some exceptions, including two attacks this past summer that the group claimed a month later.
It is possible that the group is simply taking its time.
Among the arguments for why the Islamic State would not claim responsibility, even when everything points to it, is a pragmatic consideration.
“It’s essentially about loyalty. No one would be motivated to do an ISIS attack if they know that when they end up getting caught, someone will out them,” said Raphael Gluck, an independent researcher who focuses on the group’s digital footprint and has embedded himself in many of the group’s online chat rooms. “It would work counter to their whole recruitment line and would serve to push off the next attack.”
Amarnath Amarasingam, a senior research fellow at the Institute for Strategic Dialogue, who has interviewed Islamic State followers online as part of his work, said that members of the group explained to him that not asserting responsibility for attacks was intended to protect fighters.
“I think it’s very much a matter of internal communication,” he said, “a way of telling their supporters that if they are captured, they won’t be sold out by the group.”
Others said they see a theological justification: The Islamic State encourages recruits to die in a standoff with police in a “martyrdom operation.” When the fighter survives, the mission is incomplete, Mr. Brisard said.
This theory has been given weight by several recent attacks, including the London Bridge and the Barcelona attacks this summer, in which assailants wore what turned out to be fake suicide belts. Online tutorials teach followers how to make these look-alike vests, strapped on for the purpose of inviting law enforcement’s gunfire.
That motivation may also explain why the Manhattan assailant, Sayfullo Saipov, emerged from his rented Home Depot truck armed with both a pellet gun and a paintball gun, neither powerful enough to cause significant harm, but making him look dangerous to the police.
“According to their logic, their ideology, they are supposed to die during the action,” Mr. Brisard said.
He and others discounted the notion that attacks were not claimed because they were not deemed successful. The group has asserted responsibility for foiled plots, like the attacker who rammed his explosive-laden car into a police vehicle on France’s Champs-Élysées in June.
No one was hurt except the assailant, who had appeared on France’s S-list of radicalized individuals.
He was killed, and sometime after, the Islamic State claimed the attack, calling it a reminder to Western powers that the “battle has come to their homeland.”
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