Mass murder in the United States is increasing, yet understanding of mass murderers is still relatively limited. Many perpetrators compose manifestos, which include journals, blogs, letters, videos, and other writings. Previous research has indicated that personal messages are of great social and psychological importance; however, there remains an important gap in the current literature regarding studies specific to these manifestos. As such, the purpose of this qualitative multiple case study was to provide greater understanding of mass murderers' motives and mindsets through psycholinguistic analysis of their recorded words. The constructivist conceptual framework enabled gathering, analyzing, interpreting, and reporting thematic language from a purposeful sample of 12 American mass murderer manifestos, all of which were freely available online. The 6 research questions aligned with 6 psycholinguistic themes: ego survival and revenge; pseudocommando mindset: persecution, envy, obliteration; envy; nihilism; entitlement; and heroic revenge fantasy. Descriptive and analytical coding allowed for the identification of sentences and passages representative of each theme. Findings revealed a high degree of support for nihilism and ego survival and revenge, moderate support for heroic revenge fantasy and pseudocommando mindset, and limited support for entitlement and envy. These findings contribute to the existing literature, enhancing social change initiatives through increased understanding of mass murderers' communications and prompting further needed research. With greater awareness comes the potential for early identification and intervention, which may favorably impact psychology and law enforcement professionals and at-risk individuals. [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.]