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 the 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.