Washington- In the summer of 2015, armed American drones over eastern Syria stalked Junaid Hussain, an influential hacker and recruiter for ISIS.
For weeks, Mr. Hussain was careful to keep his young stepson by his side, and the drones held their fire. But late one night, Mr. Hussain left an internet cafe alone, and minutes later a Hellfire missile killed him as he walked between two buildings in Raqqa, Syria, ISIS’ de facto capital.
Mr. Hussain, a 21-year-old from Birmingham, England, was a leader of a band of English-speaking computer specialists who had given a far-reaching megaphone to ISIS propaganda and exhorted online followers to carry out attacks in the West.
One by one, American and allied forces have killed the most important of roughly a dozen members of the cell, which the FBI calls “the Legion,” as part of a secretive campaign that has largely silenced a powerful voice that led to a surge of counterterrorism activity across the United States in 2015 as young men and women came under the influence of its propaganda.
Carrying out one strike after the next, American and British forces set out to destroy the Legion. Reyaad Khan was also killed in a drone strike.
An Australian, Neil Prakash, was targeted in a strike around the same time. A senior American official said that Mr. Prakash was wounded but survived. In the last few weeks, however, a Middle Eastern government arrested Mr. Prakash, another senior American military official said.
American military, intelligence and law enforcement officials acknowledge that ISIS still retains a sophisticated social media arm that could still inspire attacks like those in San Bernardino, Calif., and in Orlando, Fla., and remains a potent foe suspected of maintaining clandestine cells in Europe. But they point to the coordinated effort against the Legion as evidence of the success the United States has had in reducing ISIS’ ability to direct, enable or inspire attacks against the West.
Initially the threat posed by the Legion was primarily seen as a problem for law enforcement officials. But as the threat worsened last year, and the FBI stepped up the monitoring of terrorism suspects around the country, the bureau pressed the military to focus on the group, according to current and former American officials.
While American and British forces conducted a series of drone strikes on members of the group, the FBI sifted through thousands of the Legion’s followers on social media to figure out who had actually been inspired to take action. In the last two years, it has arrested nearly 100 people in cases involving the terrorist group.
Several of the arrests were of people who had direct contact with the Legion. Many of the others involved were “folks who first came on our radar because we became aware of them” through their connections with Hussain and Reyaad Khan, also a British citizen, who was another leader of the group, according to Andrew McCabe, deputy director of the FBI.
Mr. Hussain wore a number of hats, including that of a hacker. He was linked to the release of personal information on more than 1,300 American military and government employees. In March 2015, his group posted the names and addresses of service members with instructions: “Kill them in their own lands, behead them in their own homes, stab them to death as they walk their streets thinking they are safe.”
More important were Mr. Hussain’s efforts as an online recruiter.
According to court records, Mr. Hussain communicated with at least four men in four states, imploring them to initiate attacks or help spread ISIS’ message. Mr. Hussain was behind a plot to behead Pamela Geller, the author of a conservative blog. In early 2015, Mr. Hussain began communicating with Usaamah Abdullah Rahim, 26, and gave him instructions to kill Ms. Geller.
Mr. Rahim abruptly abandoned the plan and decided instead to kill a police officer in the Boston area. The bureau was monitoring him, and Mr. Rahim was shot and killed in June 2015 after he confronted an FBI surveillance team with a knife. The FBI also arrested two of Mr. Rahim’s associates, whom prosecutors say were involved in the plot.
Mr. Hussain’s associates were also busy. Another Briton, named Raphael Hostey, was in touch with Mohammed Hamzah Khan, 19, of Bolingbrook, Ill. Mr. Khan tried to travel to Syria with his two younger siblings before he was arrested by the FBI.
In another plot that the FBI disrupted, Mr. Hussain instructed an Ohio college student named Munir Abdulkader to kidnap a member of the military and record his killing on video. Mr. Hussain then asked Mr. Abdulkader to attack a police station in the Cincinnati area. As Mr. Abdulkader prepared for the suicide operation, he told Mr. Hussain about his prowess on the shooting range.
Mr. Hussain responded: “Next time ul be shooting kuffar in their face and stomach.”
Mr. Abdulkader, 22, who was born in Eritrea, was arrested and pleaded guilty in July to material support for terrorism and plotting to kill a member of the military and police officers. He was sentenced to 20 years in prison.