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In the last twelve months it has attacked government websites in Syria, declared cyber war on a brutal Mexican drug cartel, and exposed an anti-WikiLeaks "dirty tricks campaign" allegedly plotted by a prominent US security firm. But on Friday, Anonymous, a diffuse network of internet hackers, reached a new level when it intercepted and leaked a conference call between FBI agents and Scotland Yard detectives.

The astonishing feat - confirmed as genuine by the FBI - was apparently carried out after the hackers breached email accounts belonging to the authorities. In doing so, they were able to snoop on communications being exchanged between forces involved in a joint international anti-hacking operation across England, Ireland, Holland, France, Denmark, Sweden and America. In a piece of surreal real-life theatre, the tables were embarrassingly and dramatically turned. The investigators became the investigated; the watchers became the watched.

The call in question, which lasts around 16 minutes, is one of the boldest leaks ever produced by the hackers, and it may also be one of the most revelatory. A fascinating glimpse into a highly classified world, it shows the extent to which the Metropolitan police is willing to collaborate with its foreign counterparts as part of cyber-crime investigations, even if doing so means interfering with the British judicial process. At one point during the call, for instance, one of the Scotland Yard detectives tells his FBI colleagues that they secretly delayed an ongoing court case involving two UK-based suspected hackers - Jake Davis and Ryan Cleary - at America's behest.

"Following some discussion with the New York office, we're looking to try and build some time in to allow some operational matters to fulfil on your side of the water," the Scotland Yard detective is quoted as saying. "We've got the prosecution making an application in chambers, i.e. without the defence knowing, to seek a way to try and factor some time in, that won't look suspicious." He goes on: "Hey, we're here to help. We've cocked things up in the past, we know that."

The FBI has previously declined to comment on whether it would pursue extradition of Cleary or Davis, both of whom are facing a series of charges in Britain for their alleged involvement with Anonymous and its affiliated offshoot, LulzSec .

The call suggests, however, that the US could indeed be building its own case against the hackers. Davis in particular, who stands accused of being the audacious LulzSec spokesperson known online as "Topiary", would no doubt be wanted by the Americans. Over a two-month period in 2011, LulzSec perpetrated a series of high-profile attacks on the websites of US-based multi-national corporations and state agencies - including the CIA and the US senate - making it a prime target for cyber-crime investigators within the FBI.

Prior to the leaked call, it was clear that Davis's legal team already suspected US involvement on some level. This was made apparent last month, during a short hearing at Southwark Crown Court, when Gideon Cammerman, Davis's lawyer, expressed concern about outside interference, asking prosecutors that any "letters of request from a foreign jurisdiction" are presented to him when evidence is formally exchanged on 30 March, prior to Davis and Cleary entering pleas on 11 May. (A letter of request is a method used by a foreign court to seek judicial assistance, such as to obtain information or a witness statement from a specified person.)

Responding to concerns raised by Cammerman, a source within the Crown Prosecution Service said that they could not officially comment on the matter of foreign involvement until after 30 March, but stressed both prosecution and defence had a "common interest in the case being tried here [in the UK] effectively," hinting that any possible US extradition request could hinge on the outcome of the British trial.

In the meantime, the key question is whether Anonymous is sitting on more hacked information as explosive as the conference call, which, depending on its content, could have potentially massive repercussions.

To some extent, the authorities on both sides of the Atlantic have now been put on the back foot. Likely rattled and aghast that their own private conversations were hacked by the very hackers they are paid to investigate, they will be apprehensive about what could come next.

Cleary's lawyer, Karen Todner, has starkly warned that "whole cases could be blown apart" as a result of future security breaches; Anonymous, as ever, has promised more revelations are yet to come.

"You think we're done? Fuck no," tweeted one of its most prominent hackers, Sabu, on Friday. "Truth is we're still in the agents (sic) mailbox right now."

Ryan Gallagher is a freelance journalist based in London. His website is here