WASHINGTON — Some time last year, a Kuwaiti man in his early 30s who had spent more than a decade hiding from the American government arrived in northwest Syria, where he met up with other members of Al Qaeda who had begun putting down roots in a country torn by two years of death and chaos.

American intelligence officials believe that the Kuwaiti, known sometimes as Muhsin al-Fadhli, had been sent from Pakistan by Ayman al-Zawahri, Al Qaeda’s leader, to take over a cell that could one day use Syria as a base for attacks in Europe and possibly the United States.

Unlike other jihadist groups that have come to prominence in recent years, the cell that Mr. Fadhli came to lead — known within intelligence and law enforcement agencies as the Khorasan Group — avoided the spotlight. It put out no slick Internet magazines and did not boast of its plans on Twitter.

The group’s evolution from obscurity to infamy has been sudden: The first time President Obama publicly mentioned the group was on Tuesday, when he announced he had ordered an airstrike against it to disrupt what American officials said was a terror plot aimed at the West.