A vast online community has formed over the last decade around a belief in gang-stalking, with "targeted individuals" sharing stories of mobs of stalkers, mind games, thought control, and extreme surveillance being used to destroy their lives.

In "gang-stalking," everything seems connected, and inconsequential details acquire new purpose. That person who crossed your path earlier. That siren outside your window. That chair in the kitchen—is it where it was before? Has someone been in your house, moving things around? Are there microchips under your skin?

Dr David Crepaz-Keay of the UK's Mental Health Foundation said he had encountered patients who fit this profile. "I can easily see how a combination of behaviours (individual and public) can create a plausible impression of gang-stalking," he said.

Media outlets pieced together Long's online presence as "Freedom Strategist" and "Mental Game Coach" Cosmo Setepenra, the pseudonym he used to create a series of e-books, YouTube videos, and a Twitter feed. Reports also connected Long to posts on a page titled "Stop Organised Gang Stalking," where a user named cosmo717 was presumed to be Long.

"It's difficult to say how widespread the problem is but it's certainly not uncommon"

Most often kept to the febrile fringes of the conspiracy web, gang-stalking appeared in mainstream news this week in reference to the Baton Rouge shooter Gavin Long, a former marine who killed three police officers and injured three others before being shot dead.

Inevitably, an e-book industry has also sprung up around gang-stalking, offering guides on " How to Deal with and Defeat Gang Stalkers " ("They attempted to stop her from publishing this book by deleting it from her computer and causing formatting problems with ALL previous editions. Multiple perpetrators are involved suspected and unsuspected") and works of fiction .

A report in the New York Times last month described gang-stalking as a community "conservatively estimated to exceed 10,000 members," dispersed across blogs, forums, Facebook groups, and YouTube. Complaints of gang-stalking run from mild nuisance to severe trauma and self-harm. Posts describe sufferers going on the run, becoming homeless, and severing contact with friends and family.

Gang-stalking fears act as a trap: The believer behaves warily in public, and people respond to this by treating them as unusual. Crepaz-Keay explained, "This behaviour reinforces the anxiety and sparks paranoia, which increases the physical and verbal reaction which in turn increase the intensity of public response. So although there is not a concerted stalking activity, it is very easy to interpret real-world behaviour as if it is co-ordinated."

The study's results were not surprising: Among the gang-stalked, all were found to likely be suffering from delusions and rated more highly for symptoms of depression, trauma, and adverse impact on social functioning. They reported feelings of going mad, depression, fear, distrust, and suicidal ideation. Their relationships broke down, they had lost jobs, and some had decided to carry a weapon. Victims were unable to identify their stalkers by name. The majority of them, curiously, were women. Overwhelmingly, they said they would not go to the police for fear of being ignored.

I spoke to one of the paper's authors, Dr Lorraine Sheridan , by email. "It's difficult to say how widespread the problem is but it's certainly not uncommon," she said.

In the study, 1,040 self-defined victims of stalking filled out an anonymous questionnaire on a website offering advice and support. One hundred and twenty-eight of the respondents reported group-stalking. Their complaints were a vivid cross-section of gang-stalking allegations, including reports of "teams of men in black vans," "everyone in the street being 'plants' acting out roles towards the victim," "'more than a thousand' people being involved,""use of 'voice to skull' messages," "witchcraft focussed through gold objects," "organised electronic mind interference," and (categorized by the researchers under "bizarre") "docile family dog replaced by exact double with foul temper" and "remote enlargement of bodily organs."

There is only one research paper specifically on the topic, written by forensic psychologists Lorraine Sheridan and David V James and titled " Complaints of group-stalking ('gang-stalking'): an exploratory study of their nature and impact on complainants ." It was published in 2015 in The Journal of Forensic Psychiatry and Psychology. The tags attached to the paper are telling: "stalking," "group-stalking," "victims," "delusions," and "post-traumatic disorders."

The damage caused by delusions of gang-stalking is progressive, until it consumes every moment of a sufferer's life. Some become jobless and homeless, though others are just about able to continue functioning as before. "In these cases, friends and family of the individual often suffer the most," said Sheridan. "Some people even profit from their beliefs, making lots of new friends and contacts via the internet and becoming campaigners/speakers/writers focused on educating people about gang-stalking."

Gang-stalking also borders on a fear of technology itself

Since the study's publication, mental health professionals have contacted Sheridan to comment on how hard gang-stalking beliefs are to eradicate in their patients. Members of the community have sent threats and accused her of being paid by the government (for the record, Sheridan says nobody paid her to produce the study).

Sheridan believes that the idea of gang-stalking has always existed as a delusion, but that the internet has helped it to grow. "The internet creates a sort of 'closed ideology echo chamber' wherein people who share unusual beliefs reinforce each other's thinking. There is no counter-argument within these groups, they are like minds," she said.