Mostly Human

"I think that was pretty much what made him [so] dangerous [was] the ability to tell a good narrative and be compelling," Nichols said.

What made that narrative different than other terrorist organizations was that it was inclusive. Nichols compares it to the hacker group Anonymous, where Hussain initially found belonging.

"It's a decentralized network where all you have to do to be Anonymous is say ‘I'm Anonymous,'" he said. "All you have to do to be ISIS is say you're ISIS...you'll find a community and you'll find support. A lot of the other terrorist groups were more centralized and their narrative was more tightly controlled."

Nichols said Hussain took advantage of the many disaffected people online, sharing an alternative viewpoint of ISIS' motivations.

"[We say], 'You shouldn't join ISIS because they chop off heads,'" Nichols said, "And they go, 'Well, I shouldn't join the West because there's decades of history of you screwing us and you bombing our wedding parties from the sky.' From that perspective, which is the greater Satan there?"

With TriCk running social media, ISIS became more accessible than ever, and U.S. authorities started connecting Hussain to conspiracies to attack the United States. They also linked him to an attack in Garland, Texas, where a U.S. official said Hussain may have played a role in inspiring the gunman.

When Twitter would shut down Hussain's account, he'd log in under a new name. It was a game of whack-a-mole. Hussain also reached out to his followers on encrypted apps, becoming increasingly difficult to trace.

Government officials were forced to grapple with the new reality.

"It's the most complex terrorist threat we've faced, period. We've seen terrorist groups use social media platforms to bombard vulnerable populations, kids who may be on the fringe," Carlin said. "They just want to figure out a way to turn you into a human weapon."

According to Josh Corman, a security expert who followed Hussain's role in ISIS, the terrorist organization knew the value of his cyber skills.

"One of the more disturbing things I learned was that ISIS was paying dollar for dollar the same payroll for someone training combatants as they were for someone doing social media," said Corman. "That just shows you how important it is to them and how much they treasure and value the recruitment capabilities of web pages and social media."

The tipping point came after what seemed like a basic hack. Ardit Ferizi, a hacker from Kosovo, breached the server of a U.S. retail company and stole names and addresses of thousands of customers. Ferizi combed through the data to identify 1,300 government workers and handed the list over to Hussain, who used them to create a "kill list." He blasted it out on Twitter with the message "kill these people where they live."

The list went viral.

"When you're in a place where you can't otherwise be captured and you pose an imminent threat to life, we're going to take actions for self defense," Carlin said.

It wasn't long after that the government made the decision to kill Hussain.

Hussain's neighbor, who asked not to be named, remembers the day his family learned about his death.

"It was horrendous. I know he'd done whatever he'd done, but I heard the grief next door, and I thought ‘Oh my god, he's dead. He must be dead.'"

She held Hussain's mother.

“It wasn’t the mosque, it wasn’t his family. It was the computer," she said. “Other people locally have no sympathy. But it could be anybody’s son.”

Hussain's legacy lives on. The U.S. government continues to grapple with social media-inspired attacks and encrypted communications.

Carlin says the game has changed as authorities face a new challenge -- sorting through the people online who talk about violent extremism and those who actually plan to commit acts of terror.

"They have to pick out who's just curious, who's on the road but not there yet, and who's about to go outside the door with a knife or a gun and kill their neighbors," he said.

It's a problem that gets harder in a digital age where there's much less time to intervene.

"If you think about a stick of dynamite and you light the fuse, it used to be there was a long time down that wire before the dynamite goes boom," Carlin said. "Now, with these young and unstable individuals, it may be a very short flash, so you have a very limited window of time to figure out who that is and take action."

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Produced by Erica Fink, Laurie Segall, Jason Farkas, Justine Quart, Roxy Hunt, Tony Castle, AK Hottman, Benjamin Garst, Haldane McFall, Gabriel Gomez, BFD Productions, Jack Regan, Cullen Daly.

Article edited by Aimee Rawlins.

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