Some 54,000 Twitter accounts were reportedly hacked Sunday night by members of the Islamic State militant group, also known as ISIS, who sought revenge for the killing of one of the group’s lead hackers earlier this year. The hacker team, which goes by the name Cyber Caliphate, released phone numbers they said belonged to the CIA, FBI and NSA chiefs. They said it would take years to post all the information they managed to gather, the Middle East Eye reported.

Junaid Hussain, a British ISIS recruit who encouraged supporters worldwide to use Twitter to spread ISIS propaganda, was killed in a joint U.S.-U.K. drone strike in August. The Pentagon listed him as the third most important target on their "kill list." His wife, Sally Jones, remains on the government’s list of most dangerous ISIS recruits.

Most of the victims of Sunday evening’s hacks were based in Saudi Arabia, although some were also believed to belong to British citizens. "We need years to publish what we have. We will raise our flag in the heart of Europe," the group reportedly declared on Twitter.

From their strongholds in Syria and Iraq, ISIS has turned toward online social media outlets like Twitter and Facebook to gather a steady flow of new recruits from around the world. The Cyber Caliphate was responsible for hacking into the U.S. Pentagon’s Twitter account in January, but had gone mum after Hussain’s death.

"We are back,” the Twitter hackers said.

Twitter shut down an account that threatened to release more information that was obtained during Sunday's hacking.

“Users cannot [use Twitter to] threaten or incite violence, which includes promoting terrorism,” a spokesperson for Twitter said.

ISIS currently rules more than about a third of Iraq and Syria, and has set up a stringent legal code based on a narrow and puritanical interpretation of religion. In areas under their control, freedoms are harshly suppressed, and death sentences have become regular. The U.S. began an airstrike campaign against the group in June 2014, shortly after militants took over Iraq's second largest city Mosul.