FBI manhunt via social media? Citizen sleuthing is a double-edged sword.

A cache of explosives in a San Francisco apartment prompts an FBI warning and request for help. The missing man posts an apology to friends. The public responds to all of it, posting and Tweeting.

After reportedly discovering a cache of explosives in the San Francisco apartment of Ryan Kelly Chamberlain, the FBI asked the public for its help in tracking him down, initiating a nationwide manhunt. The 42-year-old political and media consultant dropped out of sight over the weekend. Law enforcement warned he should be considered armed and dangerous.

The public responded, posting and Tweeting.

Then on Monday, an open letter from Mr. Chamberlain, written in advance but set up for automated release, was posted, detailing his lifelong struggle with depression, love, and work and suggesting that he was at the end of the line. A subsequent letter, reacting to the FBI’s weekend warnings but also posted Monday, disputed the notion that he was a threat.

Monday’s postings have prompted even more online dialogue from those who knew him as well as interested strangers, providing an avalanche of opinions, ideas, and information about his activities and personal taste.

Some of this may eventually prove useful to law enforcement, say experts. But this barrage of social media in the midst of an active manhunt points out the growing positive as well as negative impact of participatory real-time, citizen sleuthing.

“We all love good stories, and a real-time criminal investigation in which we can take part is as good as it gets,” says Janet Johnson, assistant professor of social media communication at the University of Texas at Dallas.

However, Professor Johnson notes that the history of mass media is rich with the tragic and near-tragic consequences of mixing media and police work. Take the 1972 Munich Olympics at which Palestinian terrorists took Israeli athletes hostage. The gunmen were able to outmaneuver the police as they watched their stakeout covered on local television. All the hostages were eventually killed.

Fast forward to the modern era with citizen social media thrown into the mix. Law enforcement’s pursuit of the Boston Marathon bombers in April 2013 showed the power of social media to provide new clues – police retrieved roughly a million videos and photos that helped track the suspects down.

But, the headlong determination of the public to help out also led to postings on Reddit fingering the wrong man.

Johnson notes that the social media outlet learned its lesson – after the subsequent Washington Navy Yard shooting that September, Reddit immediately shut down its sub-community “FindNavyYardShooters,” to avoid repeating the earlier embarrassment.

Chamberlain, a professional social media contractor, has helped feed the public’s social media response with his carefully timed postings, points out Charles Palmer, executive director of the Center for Advanced Entertainment and Learning Technologies at Harrisburg University of Science and Technology in Pennsylvania.

Chamberlain may have been nominally posting to his own circle of social media friends, apologizing and explaining himself, but the messages were crafted after the FBI searched his home. As a pro, he would know the potential reach of any online posting, and of course, says Professor Palmer, this is intentional. “He is looking for sympathy and understanding, for his experience,” he adds.

Social media is permanently in the mix of any investigation from now on, says Fordham University media professor Paul Levinson, author of “New New Media.” The new technologies enable participation in a way that is almost unimaginable even a generation ago, he says, “which is a double-edged sword.”

On the one hand, he points out, such public participation provides incredible reach. But, the pitfalls are only just now emerging. Police were having a hard time doing their job during the Boston bombing investigation, he says, because people were hanging out on street corners trying to photograph their work as they did it.

But, he adds, it is not going away. “We are going to have to learn to live with it,” he says.