The FBI released photos of two Boston bombing suspects Thursday -- and warned against publishing other images, in a clear slap to an army of amateur sleuths trying to solve the case, many online.

Cyber-detectives have taken to social media en masse since the bombing, sharing and analysing a tsunami of photos and videos available from cellphones, cameras and TV coverage of the bombing, near the Boston marathon finish line.

Taking the lead from the official investigation, the online manhunt notably focused on people with black rucksacks, posting and highlighting photos of a string of potential suspects -- sparking warnings of vigilantism.

"We do worry that it could do real harm to people if the crowd misidentifies someone and results in attacks or violence against them," Cindy Cohn of digital rights non-profit group the Electronic Frontier Foundation (EFF) told AFP.

Images of Blue Robe Guy, Running Away Guy, Brown Sweatshirt Guy and others have been analyzed in minute detail on sites including Reddit and 4chan, which had one album viewed by more than 2.9 million people by Thursday.

The pictures also included those published on the front page of the New York Post, of two men it said were being searched for in connection with the Boston bombings but who turned out to be completely innocent.

"It's the worst feeling that I can possibly feel -- I'm only 17," Salah Barhoun, one of the pair, told ABC news, which said it found the teenager through social media.

Rick DesLauriers, Federal Bureau of Investigation (FBI) chief in Boston, made clear his frustration at the proliferation of amateur sleuthing that filled the vacuum of information from real investigators until Thursday.

Unveiling photos and videos of two suspects with black backpacks, one of whom was seen placing a bag at the scene of one of the blasts, he said: "These images should be the only ones... that the public should view to assist us.

"Other photos should not be deemed credible. They unnecessarily divert the public's attention in the wrong direction and create undue work for vital law enforcement resources," he added.

One of the main forums for online Boston bomber sleuths was reddit, where a subreddit -- a space for discussion on a particular subject -- called findbostonbombers has been used all week to exchange comments and images.

Within minutes of the FBI request, an announcement appeared on the page saying: "At this point in time the only photographs that are allowed to be posted in this subreddit are images that may contain the FBI's two suspects.

"All others will be deleted."

That didn't amount to complying fully with the FBI request, since it left sleuths open to post pictures they thought "may" show images of the two FBI suspects, but it will no doubt be welcomed by investigators.

But the fact remains that, in the age of social media and ubiquitous camera phones, it is difficult, if not impossible, to keep a lid on speculation or correct misinformation.

That has been demonstrated abundantly in Boston, including early on this week when reports of a Saudi suspect being in custody turned out to be false and based on a Saudi national being among those injured.

The EFF's Cohn was cautious about the Boston online efforts. "I don't think we know yet whether crowd-sourced investigation like the Reddit one can work, since this is really very new," she told AFP.

"Real law enforcement is a skill, and even they get it wrong sometimes with horrible results for those affected," she said, citing cases of mistaken identity at the 1996 Atlantic Olympics and the 2004 Madrid bombings.

"So it's important that amateurs trying to help exercise discretion and avoid jumping to conclusions."