Nobody believed him. His family told him to get help. But Timothy Trespas, an out-of-work recording engineer in his early 40s, was sure he was being stalked, and not by just one person, but dozens of them. At first, Mr. Trespas wondered if it was all in his head. Then he encountered a large community of like-minded people on the internet who call themselves targeted individuals," or T.I.s, who described going through precisely the same thing. The group was organized around the conviction that its members are victims of a sprawling conspiracy to harass thousands of everyday Americans with mind-control weapons and armies of so-called gang stalkers. The goal, as one gang-stalking website put it, is "to destroy every aspect of a targeted individual's life." The community, conservatively estimated to exceed 10,000 members, has proliferated since 9/11, cradled by the internet and fed by genuine concerns over government surveillance. They raise money, hold awareness campaigns, host international conferences and fight for their causes in courts and legislatures. Perhaps their biggest victory came last year, when believers in Richmond, Calif., persuaded the City Council to pass a resolution banning space-based weapons that they believe could be used for mind control. A similar lobbying effort is underway in Tucson.