Mass shooters who target schools and workplaces are typically insiders such as students or employees, calling into question the effectiveness of security measures and training, according to one of the most comprehensive studies of the subject.

Barriers and locks meant to block outsiders and active-shooter drills do little because most attackers already have access and are aware of the procedures, said the authors of the study, Jillian Peterson and James Densley. The university professors have created a mass-shooter database...