LulzSec and Anonymous offshoot targets 'government systems, police systems and evil corporations' in a quest to expose what it calls high-level corruption

In secretive online chat rooms, away from the glare of police, small groups of elite hackers plot attacks against multi-national corporations and governments.

But in a quest to expose what they see as a conspiracy of high-level corruption, the hackers – affiliated to cyber-activist network Anonymous – have in recent months expanded their targets, becoming increasingly unpredictable and callous in the process.

2011 was a significant year for Anonymous, both in terms of activity and evolution. The chaotic collective, born out of online messageboard 4Chan in 2003, continued to grow, partly fuelled by the social unrest that has gripped the world.

Its members helped revolutionaries in Tunisia, Egypt and Syria counter government censorship, and temporarily disabled the websites of powerful financial institutions for refusing to process WikiLeaks donations.

They have taken part in traditional protests as part of the worldwide Occupy movement and led a challenge against new laws that they say would stifle internet freedom .

At the same time, however, an aggressive and volatile faction within Anonymous has also flourished – and that is causing a degree of internal division.

Made up of a small highly skilled hacker team which carries out attacks under the name Operation Anti-Security – or AntiSec – it includes some of those previously involved with the group LulzSec, which for two months last year attacked a series of major targets including Sony and the US Central Intelligence Agency.

Unlike the majority of mainstream Anonymous participants, AntiSec's core members – numbering up to 10, with around three or four key hackers, according to sources close to the group – do not often participate in distributed denial of service (DDoS) attacks that flood websites with traffic and force them offline for a temporary period.

Announced by LulzSec in June 2011 as part of an offensive against government agencies and what it called profiteering gluttons, AntiSec's elite – like a clandestine special-forces wing of Anonymous – devote themselves to far more precarious and controversial activity.

The group breaks into servers, exposing security vulnerabilities while mining data – often including passwords and credit card numbers – that it ultimately dumps onto the web for anyone to download.

Partly inspired by a 13-year-old hacking movement of the same name, since December 2011 AntiSec has embarked on a seemingly unstoppable rampage.

It has intercepted an FBI/Scotland Yard phone call, and attacked a well-known thinktank, a number of US police forces, a law firm and even US consumer watchdog the Federal Trade Commission. Almost nothing, it seems, is off limits.

"Generally we target government systems, police systems and evil corporations," says an AntiSec hacker, who asks not to be named ("I don't need the heat"). "But law firms do usually contain a wealth of private information, and when they are representing people who are already in our crosshairs, it's fair game."

On 24 December, AntiSec broke into the website of Stratfor, a US security and intelligence thinktank that specialises in geopolitical analysis.

Aiming to expose "rich and powerful oppressors", it stole a huge 200GB cache of data from Statfor's servers, including 5m emails and 75,000 credit card numbers belonging to the thinktank's subscribers. The emails were handed to WikiLeaks, which on Monday began publishing selections of them.

But the credit card details were simply dumped online for anyone to download, leaving thousands of Statfor's customers – among them ordinary citizens who paid to receive updates on world affairs – open to exploitation by fraudsters.

There had been more unwitting victims on 3 February, when AntiSec attacked law firm Puckett and Faraj, which represented Frank Wuterich, a US soldier convicted for involvement in the killing of 24 Iraqi civilians in 2005.

The hackers were outraged when Wuterich, who admitted issuing an order to shoot first and ask questions later, avoided a jail sentence for his role in the incident, winning leniency through a plea bargain that carried no punishment beyond a reduction in rank and a pay cut.

AntiSec broke into Puckett and Faraj's servers, obtained nearly 3GB of emails (numbering tens of thousands of messages and dating back two years), and posted them onto the internet "to expose the corruption of the court systems and the brutality of US imperialism".

The trove included emails showing how Wuterich's lawyer, Mark Zaid, planned to meet Republican congressman Duncan Hunter about "making this whole case go away". But it also contained a mass of highly sensitive information from an array of cases unrelated to the Wuterich incident, such as witness statements from victims of sexual assault.

The indiscriminate release caused unease among some members of Anonymous, with a statement purportedly from a disenchanted group (for it's impossible ever to say what is officially Anonymous policy; the group's inchoate structure means there's no such thing) declaring that a silent majority was growing uncomfortable with this new and inaccurate meaning for Anonymous.

It said that the hacks are actually not the work of Anonymous: "Yet again, the corporate or government provocateurs at #Antisec have continued their smear campaign to ruin the name of Anonymous by using it for increasingly destructive action."

AntiSec, however, has remained unapologetic.

"When justice cannot be found in the courts, it will be found in the streets – or in this case, the internet," says a hacker involved in the attack on Puckett and Faraj, adding that the group still has access to a private email account used by Neal Puckett, the firm's founder.

"Generally, we do work to redact information on bystanders – for example, in all of our police attacks, we have carefully redacted prisoner/parolee information. [But] it is the firm's responsibility to protect the information of their clients."

Will there be more attacks on law firms in the future? "If law firms stick their necks out in defence of notoriously corrupt corporations – especially if it is shown that wrongdoing was involved – then yes, I'd say that could be possible."

Adopting such an increasingly militant approach has gained AntiSec a number of vocal critics, most notably among organisations that in the past may have sympathised with the wider aims of the Anonymous movement.

Alex Hanff, managing director at consultancy firm Think Privacy and former communications project leader at civil liberties watchdog Privacy International, denounced the group earlier in February after it posted the source code of popular Symantec anti-virus software online, rendering it potentially unsafe.

Hanff, who keeps in regular contact with those involved in Anonymous actions, was asked to endorse the Symantec release by AntiSec. When he refused, saying it could make thousands of computer users vulnerable to criminals, a Privacy International affiliated website came under attack.

"People who hold Anonymous's cause close to heart are incredibly aggravated by the actions of AntiSec," Hanff says. "There is a very significant lack of social responsibility, morals and ethics – they seem to be a group that are bent on only pursing their own course, for their own purposes, without any awareness or even any care for what the consequences of that may be."

Some believe the increasingly erratic attacks, perpetrated by small cells of hackers such as AntiSec, only serve to give governments justification for asserting greater control over the internet.

At present, though, it seems even the introduction of punitive legislation would prove futile, because the most skilled hackers – as volatile as they may be – are able to outsmart and outstep the authorities at almost every turn, and, perhaps most crucially, are committed to stop at nothing.

"I don't want come off as someone who's saying that our particular grievance is going to be the thing that ruins America – that finishes it off – but that's what it's looking like from our standpoint," says Barrett Brown, an activist who works closely with AntiSec.

"This is going to turn into an actual shooting war. It just looks to me like things are accelerating ... if you're mad at us now, just wait a couple of years. No one's burning down villages; no one's dropping bombs – yet."