A British hacker believed to be a top cyber expert for the Islamic State group has been killed in a U.S. drone strike, sources reportedly said Wednesday. Junaid Hussain, a British citizen from Birmingham, reportedly traveled to Syria in 2013.

The 21-year-old was reportedly killed in the drone strike, which likely involved the U.S. Defense Department, a U.S. source told Reuters. The strike was conducted Tuesday near the Syrian city of Raqqa, a CSO Online report said. However, U.S. authorities have yet to officially announce Hussain’s death.

U.S. and European government sources told Reuters that Hussain was believed to be the leader of the CyberCaliphate, a hacking group, which in January attacked a Pentagon Twitter account. The sources could not confirm if Hussain was directly involved in the hack. The hackers took over Twitter and YouTube accounts belonging to U.S. Central Command, or CentCom, often used to provide updates about airstrikes against ISIS. 

A U.S. official called the drone strike on Hussain's vehicle Tuesday a great "intelligence success," according to CNN.

While U.S. sources were confident Hussain was killed in the strike, U.S. intelligence experts reported, citing two Twitter accounts linked to ISIS, that his wife had said he was still alive.

The U.S. government has conducted drone strikes against ISIS leaders and militants for more than a year, and has recently stepped up those efforts. Hussain's death was the second reported killing of a senior ISIS member in the last eight days after the group's second-in-command, Hajji Mutazz, was killed by an American military airstrike in Iraq on Aug. 18.

According to some reports, Hussain was recently listed as the third most important target on Pentagon's "kill list" of ISIS targets. However, U.S. sources denied the reports stating that there were many other ISIS members far more dangerous than Hussain, Reuters reported.

In Britain, Hussain was part of the hacker group Team Poison. The group had gained access to the address book of former Prime Minister Tony Blair and published information from it, for which Hussain was jailed for six months in 2012.