The reasons for committing to active shooter drills and exercises are based on psychological survival and decision-making research, along with experiences and lessons learned. However, in the realm of education, the origins and research-based purposes of active shooter drills are mostly unknown. Therefore, the aim of this study was to examine the content of active shooter practices, guidance, and training materials used in K-12 Public Education. Guidance and materials from federal, state, and local government agencies and private and not-for-profit organizations and professional associations from the 1990s to 2022 were analyzed. The study's conclusions demonstrated a link between the provision of advice on active shooter preparedness for K-12 schools almost exclusively in reaction to a school shooting tragedy that is founded on anecdotal experiences rather than empirical research and continues to be influenced by companies, organizations, and political agendas. However, since Columbine, messaging has evolved from the single response of a "lockdown" toward a multi-option, trauma-informed survival skill response. Despite not having a federal standard, many states make it a requirement considering the cognitive research that indicates the value of safety preparedness to increase chances of surviving by overcoming fear and denial, as well as the ability to physically and mentally perform practices acquired from training. However, with little to no implementation guidance, some drills are conducted erroneously, mimic actual armed assailant events, and cause trauma. Through an examination of the concepts in the training materials, this study contributes additional information for school district administrators and stakeholders to reflect on to ensure that current guidance in the field of education is in line with the recommendations that law enforcement officials and emergency management professionals believe will protect the learning environment and keep schools safe. These findings could enlighten and inform stakeholders of American K-12 schools when determining how to execute active shooter training considering the evolution of "lockdown" drills from their beginnings to the current implementation of trauma-informed, options-based exercises and drills based on cognitive research that contribute to making learning environments safer. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]