Commissioner Bob Paulson told reporters Friday in Calgary that his heart goes out to the family of 17-year-old Rehtaeh Parsons, calling it a "terrible set of facts."
The Nova Scotia girl's family said she hanged herself last week after she was sexually assaulted by four boys and a photo of the alleged incident was distributed online.
RCMP concluded no charges could be laid due to lack of sufficient evidence, but said Friday they would reopen the case because they got new information.
"We're very concerned about vigilante response to the case in Halifax. We had a similar set of facts going in British Columbia a little while ago. That's not the way to get to the bottom of these things," Paulson said.
"I think it's serious in this case because it seems to be mounting in its momentum."
An anonymous hacker group has tweeted that it knows the identities of the boys allegedly involved in the sexual assault and threatened to make that information public.
The same group was involved after the suicide last year of B.C. teenager, 15-year-old Amanda Todd, whose YouTube video described how she was sexually exploited on the Internet and then bullied.
"These are deep-rooted problems in our communities...troubled young people," said Paulson.
"I can point to the British Columbia case where some of the people were identified publicly that had nothing to do with that case. We have to be very, very careful."
Paulson said he would be open to working with the anonymous group but isn't optimistic that will end up happening.
"In some circumstances they present themselves as a helpful sort of civic-minded enterprise but in other cases they don't. And they don't have the same sort of stringent review of evidence and concern for truth that we have developed in our efforts," he said.
"If they want to work with us they've got to take their mask off, sadly, but I don't think they're prepared to do that."
Mounties in Halifax said they were reopening their investigation after a person provided new information.
RCMP said the person is willing to verify who they are, the reason they're providing it and work with investigators.
Police also said the information did not come from an online source.