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Last August, a drone strike eliminated Junaid Hussain, a man who was the top hacker in the ranks of ISIS. Now, the terror group vowed to avenge his death.

CyberCaliphate, an ISIS-affiliated hacking group, posted an image with a pledge to take revenge against the United States for the death of Hussain on the group’s official account on the encrypted messaging app Telegram.

A screenshot of the post was first identified by the Middle East Media Research Institute (MEMRI), who forwarded it to The Hill.

The image is headed by the words “Gazwa” (a holy battle for the expansion of Muslim territory), “Revenge,” and “Abu Hussein,” superimposed over a picture of armed ISIS fighters holding the flag of the self-proclaimed caliphate. The bottom of the image bears the words “Islamic State /HD” and “Caliphate Cyber Army.”

revengeJunaid Hussain, a British-Pakistani hacker who was previously jailed for hacking Tony Blair’s accounts, left the United Kingdom in 2013 to join ISIS in Syria. There, he went by the nom de guerre of Abu Hussein al-Britani.

His most prominent action involved the release of personal information of over 1,300 US government and military personnel. He was also believed to be the head of the CyberCaliphate.

Hussain was responsible for improving ISIS’ digital security efforts, including defences against western surveillance and securing ISIS’ leaders’ communication systems. He also created hacking tools for ISIS operatives and helped recruit lone wolf attackers around the world. The pledge did not include a specific threat on who or when would be attacked.

“Steven Stalinsky, executive director of MEMRI, said the last few CyberCaliphate posts vowing similar revenge did not lead to significant hacks or data leaks. In some instances, ISIS hackers simply re-posted information on U.S. officials they had already leaked,” reports The Hill.

With the elimination of Abu Hussein, the CyberCaliphate may have had its ability to perform advanced hacks seriously curtailed. Whether they launch a successful operation to avenge his death could help western intelligence determine how seriously their capabilities were damaged.