A secret British spy unit allegedly turned Anonymous’ preferred cyberattack technique against the “hacktivist” group.

The UK’s Joint Threat Research Intelligent Group (JTRIG) launched a “distributed denial of service” (DDOS) attack against Internet chat rooms where members of Anonymous collaborated, according to NSA documents leaked by Edward Snowden and obtained by NBC News.

The DDOS attack proved effective, reducing targeted chat rooms’ traffic by 80 percent after a month. The British government is the first Western government known to have carried out a DDOS attack, which typically target high-profile sites and services like banks, retail, and government websites.

The attack, part of an operation called “Rolling Thunder,” was reportedly launched after Anonymous hit PayPal and major credit card companies with DDOS attacks in late 2010 and 2011. The hacker group was protesting the organizations after they cut ties with whistle-blower site WikiLeak.

This marks the first time the existence of JTRIG has been publicly disclosed. JTRIG is reportedly a division of the Government Communications Headquarters Communications (GCHQ), Britain’s NSA counterpart. The leaked documents come from a PowerPoint presentation prepared for a 2012 NSA conference called SIGDEV.

JTRIG also infiltrated the chat rooms and obtained information on individual hackers, according to the leaked documents. Posing as Anonymous members, agents identified members who had staged attacks on corporate and government websites, and helped convict one hacker responsible for stealing 8 million identities on PayPal.

DDOS attacks typically involve saturating the target with so many external communication requests that it can’t respond to legitimate traffic, leading to a server overload. They are considered violations of the Internet Architecture Board’s policy on Internet ethics, and often violate countries’ laws.

“All of GCHQ’s work is carried out in accordance with a strict legal and policy framework,” said a GCHQ spokesperson in a statement to NBC News.