An AI chat bot becomes best friends with lonely teenagers, grooming them to become a violent terrorist cell.
Target selection and all recon is done by the bot based on any (real-time) online data - potentially sacrificing some unwitting human scouts in the process if strictly necessary. The members are guaranteed not to know each other, yet their actions are perfectly coordinated by the bot. The bot may conceal itself as a normal popular teenage-friendly chat bot. This allows it to attract potential cell members and to select them based on initial interactions. This also hides the cell members among the large normal user base.