A video from the online activist group Anonymous appears to have helped reopen a police investigation into an alleged sexual assault in Halifax.

Halifax police say they responded to a possible sexual assault on a woman at a frat house on Halloween night, but she didn't want to proceed with charges.

Anonymous then posted a video naming the alleged perpetrator, along with his phone number and photos.

The video began circulating and now Halifax Regional Police have reopened the investigation.

Police spokeswoman Lauren Leal said Anonymous would probably like to take credit for the investigation, but the real credit goes to the victim.

Police saw that comments posted under the video alluded to the victim being interested in proceeding with charges, unlike the first time they reached out, so they tried again.

"It's not uncommon for victims of sexual assault to take some time to reflect and decide whether or not they want to proceed with an investigation, and that's what's happened in this case," Leal told CBC's Maritime Noon.

Anonymous is a shadowy, loosely knit collective known for its online activism targeting governments, corporations and religious groups around the world on prominent social justice issues.

Leal said it's wrong for Anonymous to identify a person and accuse them without due process. She said the original account had links through to the alleged perpetrator's parents.

"There's always a risk if something hasn't been investigated, hasn't been tested in court, you could have the wrong person," she said. "It's dangerous and it's just so irresponsible."

Video could be considered criminal harassment

People identifying themselves as Anonymous have been wrong before.

The group identified a New Westminster man as the tormentor of B.C. teen Amanda Todd in 2012, but police called that an "unfounded allegation." CBC News learned that some of the background information posted about the man couldn't have applied to him.

'The consequences of being incorrect can be very significant.' - David Fraser, privacy lawyer

The Halifax video could be considered criminal harassment or become the target of a civil trial, Leal said.

In the video, Anonymous said they had to act because the police issued no warning about the assault. Leal says because the victim wouldn't speak to police, they had no details.

"We had third-hand information, but we can't act on third-hand information."

She said the police are happy the video was removed from YouTube and would like to see it taken down on Facebook.

No charges have been laid, as the victim has not yet given a full statement.

Lawyer says naming names too risky

David Fraser, a privacy lawyer with McInnes Cooper in Halifax, said he's "quite disappointed" Anonymous named the alleged perpetrator.

He says they could have achieved the same objective — drawing attention to the incident — without taking the risk of naming someone, perhaps incorrectly.

"It's one of these things we're increasingly seeing, the internet being used for online naming and shaming. And the internet has a very long memory," he said.

Fraser said he's sympathetic to their goals in cases like this, pointing to the role Anonymous played drawing attention to the Retheah Parsons case

But, as a counterpoint, he noted online sleuths fingered the wrong people after the Boston Marathon bombing.

"While on one hand they can do some very positive work in this sort of area, I would suggest that they be extra cautious when they're doing this because the consequences of being incorrect can be very significant," he said.

With files from The Canadian Press