Junaid Hussain, a hacker, was a leading member of the CyberCaliphate, an Islamic State unit that broke into the U.S. Central Military Command’s Twitter and YouTube accounts.

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LONDON — A 21-year-old hacker from Birmingham, England, who tapped into U.S. military networks and was a central figure in the militant Islamic State group’s online recruitment campaign, has been killed in Syria by a U.S. airstrike, according to three senior U.S. officials.

The hacker, Junaid Hussain, was a leading member of the CyberCaliphate, an Islamic State unit that broke into the U.S. Central Military Command’s Twitter and YouTube accounts this year. He was considered the second-most prominent British member of the Islamic State group, after Mohammed Emwazi, a fighter often referred to as “Jihadi John” because of his role in the videotaped killings of Western hostages.

The U.S. officials, speaking on condition of anonymity, said Hussain had been killed in an airstrike Tuesday outside Raqqa, Syria.

The news of Hussain’s death comes as the Obama administration is debating the effectiveness of the U.S.-led military coalition’s campaign against the Islamic State group. The Defense Department’s inspector general is looking into whether military officials have skewed intelligence assessments to present a more optimistic picture.

Hussain’s unit has been credited with the Islamic State group’s adept manipulation of social media to recruit fighters and spread propaganda, and his online activity was increasingly linked to plots carried out far from the battlefields in Syria and Iraq, experts said.

Hussain, who was believed to use the nom de guerre Abu Hussain al-Britani, offered encouragement on Twitter to the two gunmen who staged a shooting attack in Garland, Texas, in May, at a contest for caricatures of the Prophet Muhammad.

He also used social media to threaten to raise the black Islamic State group flag over “10 Downing Street and the White House,” referring to the official residence of the British prime minister and the president.

In June, he was linked to a plot to attack an Armed Forces Day parade in South London using a bomb rigged in a pressure cooker, similar to the one used in the Boston Marathon attacks in 2013. The plot was thwarted after Hussain unwittingly revealed details to an undercover reporter from The Sun, a British tabloid, who was posing as a potential recruit.

Hussain was prosecuted for hacking in Britain in 2012 and was convicted on charges that he illegally gained access to former Prime Minister Tony Blair’s address book the year before. He spent six months in prison.

Afterward he was arrested again, on a charge of violent disorder, and while free on bail moved to Syria in 2013.

He was joined there by his wife, Sally Jones, 45, a former punk-rock musician from southeastern England who met Hussain online.

Hussain took his hacking skills to the Islamic State group. He helped instigate online attacks, posted information about U.S. military forces and officers, and acted as a cheerleader for other fighters in the group, said Raffaello Pantucci of the Royal United Services Institute in London, a research organization specializing in defense and security.