He was widely thought to be a leading player in the Cyber Caliphate, an Islamic State hacking unit linked to attacks that hijacked the social media accounts of news outlets and the U.S. military. The Islamic State is also known as ISIS or ISIL.
Hussain was also known as Abu Hussain al-Britani and played a key role in the group's online recruitment efforts. His online persona often appeared linked to Islamic State attacks across the world. In May, he appeared to offer online encouragement to two gunmen who opened fire at a contest in Garland, Tex., featuring caricatures of the prophet Muhammad. And in June, he was linked to a failed plot to attack an Armed Forces Day parade in London using pressure-cooker bombs.
The Islamic State operative also had a high-profile history as a hacker. As a teenager, he was thought to be a leader in a hacktivist collective known as TeaMp0isoN that was suspected in cyberattacks against NATO, the United Nations, NASA and other targets, according to CNN.
In 2012, he was sentenced to six months in prison after pleading guilty to charges related to placing nuisance calls to a British anti-terrorism hot line and illegally accessing and publishing former prime minister Tony Blair's address book. Hussain was thought to have fled to Syria in 2013 while out on bail over separate charges.