The Guardian reports that Hector Xavier Monsegur, AKA Sabu - the leader of the LulzSec group - was working as an FBI informant.
The report comes following the FBI revealing that Monsegur - who also identified himself as a member of Anonymous and Internet Feds - had pleaded guilty on 15 August 2011 in US District Court to a 12-count information charging him with computer hacking conspiracies and other crimes.
These carry a maximum sentence of 124 years and 6 months in prison. Despite his guilty plea last summer, Monsegur is alleged to have continued operating as the leader of the online activist group - tweeting as the LulzSec leader as recently as this week.
The Guardian reports that a US court document labels the FBI's informant as CW and states that he was "acting under the direction of the FBI" to help facilitate the publication of "what was thought to be an embarrassing leak of conference call between the FBI and the UK's Serious and Organised Crime Agency in February".
A second document links Monsegur with an FBI-owned computer and the release of 5 million emails taken from US security consultancy Stratfor, which are now being published by WikiLeaks.
Two Brits have been charged in connection with the operation: Ryan Ackroyd, a 25-year-old from Doncaster, and Jake Davis, a Scottish teenager who operated as Topiary and was originally arrested in August last year.
Two Irish men have also been charged. Darren Martyn, 25, of Galway, faces two charges of computer hacking conspiracy and Donncha O'Cearrbhail, 19, of Birr, Ireland, is charged with computer hacking conspiracy and unlawfully intercepting wire communications.
The case is being prosecuted by the US Attorney’s Office for the Southern District of New York. As well as the FBI, the case has also seen assistance from the PCeU; a unit of New Scotland Yard’s Specialist Crime Directorate.