Canadian police have reopened the case of a teenage girl who was allegedly gang raped after her suicide led online activists Anonymous to threaten to 'do their job for them'

Halifax, Nova Scotia, with its population of a little under 400,000, is promoted as a place devoted to outdoor pursuits, with over 150 parks. But on Sunday 14 April, a large group of people gathered outside the police station in protest against something far from wholesome. "Do your job. Do your job," the protesters chanted, in response to the death a week ago of 17-year-old Rehtaeh Parsons, who committed suicide after allegedly being raped by four boys, photographed during the attack and then viciously bullied. "Do your job," the protesters warned police, "before somebody else does it for you."

Several protesters wore V for Vendetta masks, the distinctive white masks worn by members of hacktivist network Anonymous, which had called for the protest in a bid to force Canadian authorities to review the decision not to press charges against anyone following Parsons' rape complaint in November 2011. Anonymous claims to have identified two of the alleged rapists and believes it is only a matter of time before it knows them all.

"We engaged #OpJustice4Rehtaeh in response to her suicide," Anonymous explained in a video broadcast last week. "We want the RCMP [Royal Canadian Mounted Police] to take immediate action against the individuals in question … We do not approve of vigilante action. The names will be kept until it becomes apparent you have no intention of providing justice. Better act fast."

In response Corporal Scott MacRae of the Halifax Regional Municipality, the province's local government body, said: "It's concerning … that people may be wanting to take any form of vigilante action. Opinions and rumours are being taken as a truth. Innocent people could be impacted."

MacRae also asserts that a year-long investigation into the alleged rape was carried out but the decision was taken "not to proceed with criminal charges due to insufficient evidence".

The response was ambivalent. On several blogs, Twitter and Facebook, people are calling for the boys to be named. But in the Canadian press, they're asking who made Anonymous the judge and jury. "As if contemplating a tormented child taking her own life isn't horrible enough, we must now live with online blame-mobs grabbing hold of a narrative and demanding justice, and not necessarily in a courtroom," wrote Chris Selley of the National Post.

Critics including Selley highlight the death of Amanda Todd, another Canadian teen who ended her life after being targeted by sexual extortionists, and whose alleged tormentor Anonymous outed in 2012, as evidence of what happens when "they finger the wrong culprit". The response from Anonymous was that the absence of arrests didn't mean their information was wrong but is "another example of how lazy and ill-equipped the police are in handling these types of crimes".

It's intriguing to see Anonymous involved in rape cases like this. From its involvement in Wikileaks and Scientology, to its decision to display the details of Kim Kardashian's bank account just for the "lulz", they seem to swing wildly from the pursuit of justice to the causing of chaos.

But Anonymous's involvement in the recent high-profile case of sexual assault at Steubenville High School, Ohio, in which an incapacitated 16-year-old was assaulted by two boys, while others took pictures, was something of a PR coup for the network. "I really think that the heroes in this story are the hackers who got the photos out there because I think that actually led to a kind of shaming process that pushed this trial forward," said journalist Dave Zirin. Anonymous published a video that showed the rapists revelling in their crime. "She is so raped," one boy laughed, before comparing the victim's unconscious form to "JFK, OJ's wife, Trayvon Martin". Subsequently, Ma'Lik Richmond, 16, and Trent Mays, 17 were sentenced to one year and two years respectively in juvenile detention for the assault.

If sexual assault has a ghastly new pattern in the internet age – a girl is raped, someone whips out their phone to record it, the resulting pictures aren't treated as evidence of a crime but a tool with which to taunt the victim – then Anonymous's involvement is about more than a collective sense of public anger about these assaults, it could also represent a turning point in the way local communities and the authorities deal with rape allegations.

"In these cases it is not only a question of bringing things to light, but also changing how they are seen," observes Sadie Plant, an academic with a background in the social potential of technology. "They're drawing attention to actions and power relations [that] haven't been tackled with the same urgency before. So in terms of broad tendencies, technologies of surveillance may be one side of the coin, but this kind of [reaction to] news making is clearly the other, and very powerful, counterpart."

A natural sense of excitement surrounds the subversive way that Anonymous operates, and the idea that rape victims, so often let down by the authorities, suddenly have an ally feels righteous. But critics of Anonymous maintain they are not to be trusted. Jennifer Emick, a self-described internet geek and former Anonymous member, came to virtual blows with the network when she disagreed with some of their practices and left to set up her own hacking site. She describes months of torment, from having photos of her children published online to threats of a sexual nature. "They may be crusading for rape victims, but they have also been known to persuade female members to strip and then shared the images. They are sexual bullies themselves.Don't believe that stuff about them all being equal. There are leaders, and those leaders are men. Most of them are still men."

One Anonymous member who is willing to speak to me isn't a man. Ellie, 27, lives in the north-east of England with her fiance and works "your run of the mill, soul-sapping office job". She blogs under the ironic handle Evil Anon, a play on Anonymous's public image, and has been part of the Anonymous network since 2003. "I started off on 4Chan when I was at school, just interested in memes and chatting to other people." Now she devotes her time to exposing sexual crimes. During the course of our communication over instant messenger, she sends me the IP addresses of people who are searching for images of child pornography, rape and incest. "I don't feel excluded at all from any part of Anonymous," she says, when asked whether she has experienced any sexism or bullying.

Finding out how many female Anons there are is tough: few hackers want to talk to journalists and even fewer want to identify themselves. Ellie estimates that "30-40% of the Anons I know are women".

In the end, whether Anonymous is allying with rape victims for feminist reasons or because it's good PR is irrelevant. The more important question is why it is down to Anonymous to seek justice for rape victims at all?

At Rehtaeh's funeral, the Rev John Morrell asked: "How can our society provide a safe haven for young girls? Why do young men feel that young girls are but objects for their sexual fantasies and pleasure?"

At the protest in Nova Scotia, the crowd was fairly evenly divided between men and women. One woman, Jen, took to a loudspeaker to explain why she was there. "I'm upset and I'm angry. But most of all I'm tired. I'm tired of these rape cases becoming commonplace in our society. I'm tired of these victims being let down by our justice system. The justice system failed Rehtaeh Parsons."