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 F.B.I. 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 F.B.I. 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 F.B.I.
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 the Islamic State’s 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 F.B.I. surveillance team with a knife. The F.B.I. 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 F.B.I.
In another plot that the F.B.I. 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.” Kuffar is a derogatory term for non-Muslims.
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.
And last year, the F.B.I. arrested a North Carolina man, Justin Nolan Sullivan, then 19, and charged him with trying to provide material support to the Islamic State. Federal prosecutors say he planned to target a public venue in a mass shooting. The authorities said that Mr. Sullivan and Mr. Hussain had discussed making a video of the attack for use as propaganda. When Mr. Sullivan’s parents voiced concerns about their son buying a silencer, he approached an undercover F.B.I. employee about killing them. Mr. Sullivan, who described Mr. Hussain as part of the Islamic State “cyberteam,” was also charged with fatally shooting his 74-year-old neighbor in the head.
A senior American law enforcement official described the spring and summer of 2015 as a “nightmare” for the F.B.I. A spike in terrorism activity left the F.B.I. reeling. The strain was so great, said the F.B.I. director, James B. Comey, that the bureau was struggling to keep pace with the threat, forcing it to move criminal officers to surveillance squads.
But then, carrying out one strike after the next, American and British forces set out to destroy the Legion. Mr. Hostey was reported killed in May. Several months later, Reyaad Khan was 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.
Mr. Hussain died in August 2015. His wife, Sally Jones, a former punk rock musician from southeastern England, who went with him to Raqqa, is believed to be alive. Shawn Parson, who was Trinidadian and in Mr. Hussain’s circle, was also killed.
American officials say they have been surprised that the Islamic State has failed to replace Mr. Hussain and the other members of the Legion with hackers of comparable ability. But the F.B.I. is still grappling with Mr. Hussain’s legacy.
“We are still dealing with the repercussions of that development and that recruitment of that network to this day,” said Mr. McCabe, the F.B.I. official.
An earlier version of this article gave an incorrect age for Munir Abdulkader. His is 22, not 21.