After literature review, this paper presents the largest study to date (n = 270) of psychiatric and neurological characteristics of accused murderers in the United States. This retrospective record review of pretrial detainees undergoing competency to stand trial and criminal responsibility evaluations examined demographic characteristics, psychiatric diagnosis, substance use patterns, Intelligence Quotient (IQ), and results of electroencephalogram (EEG), neuroimaging (MRI or CT) and neurological examination. Substance use and mood/adjustment disorders were common. Neuroimaging was abnormal in 18% of subjects and was associated with lower Performance IQ. EEG and neurological exam findings were not associated with measured cognitive impairment. While 16% of subjects had a FS IQ < 70, only 6% were diagnosed with mental retardation. Subjects with a psychotic disorder (p = 0.001) or an anxiety disorder (p = 0.005) were more likely to use a knife than other subjects in the study. Violence risk assessment in these patients must not only involve inquiry about firearm availability.