They have self-published dozens of e-books, with titles like “Tortured in America” and “My Life Changed Forever.” In hundreds of YouTube videos they offer testimonials and try to document evidence of their stalking, even confronting unsuspecting strangers.

“They wanted to basically destroy me, and they did,” a young mother in Phoenix says in one video, choking back tears. She lost custody of her daughter and was sent to a behavioral health hospital, says the woman, whose name is being withheld to protect her privacy. “But I am going to fight back for the rest of my life.”

She adds, “And guess what, I’m not crazy.”

Dr. Sheridan’s study, written with Dr. David James, a forensic psychiatrist, examined 128 cases of reported gang-stalking. It found all the subjects were most likely delusional.

“One has to think of the T.I. phenomenon in terms of people with paranoid symptoms who have hit upon the gang-stalking idea as an explanation of what is happening to them,” Dr. James said.

A mishmash of conspiracy theories

Perhaps unsurprisingly, the community is divided over the contours of the conspiracy. Some believe the financial elite is behind it. Others blame aliens, their neighbors, Freemasons or some combination.

The movement’s most prominent voices, however, tend to believe the surveillance is part of a mind-control field test done in preparation for global domination. The military establishment, the theory goes, never gave up on the ambitions of MK Ultra, the C.I.A.’s infamous program to control the mind in the 1950s and ’60s.

A leading proponent of that view is an anesthesiologist from San Antonio named John Hall.