COLUMBUS, Ohio—The man who gunned down nine people in Dayton, Ohio, two years ago had fantasized about mass shootings, serial killings and murder-suicide for at least a decade before carrying out the Aug. 4, 2019, attack, the FBI said Monday as it announced the conclusion of its investigation. Shooter Connor Betts didn’t share specific details with friends or family members about his fantasies, underscoring the importance of people being attuned to subtle changes in an individual, the agency said. A phenomenon known as “bystander fatigue” may account why no one reported Betts to authorities before the shooting, according to a two-page FBI summary of its report. That term is used “to describe the passivity, inaction, or inattention to concerning behaviors observed by individuals who have a close, interpersonal relationship to a person of concern due to their prolonged exposure to the person’s erratic or otherwise troubling behavior over time,” the …