Across the United States, mass murder events have been on the rise for nearly a decade. This thesis found that persons with serious mental illness perpetrated a statistically significant number of these events. Currently, law enforcement agencies are often the first and in many communities the only resource available to assist and assess mentally ill persons in crisis. This thesis investigated the current state of law enforcement training as it relates to assessing dangerousness and the risk for violence among persons with serious mental illness. It found that there is very little training and no risk assessment tool or guide currently available to assist law enforcement officers tasked with assessing mentally ill persons for dangerousness. Subsequently, this thesis examined alternative methods and models for assessing risk, including clinical violence risk assessments, and it conducted summary case studies. These included cases in which mentally ill persons committed acts of mass murder and cases where law enforcement successfully intervened and prevented mentally ill persons from carrying out planned violence. As a result of this research and analysis, a field risk assessment guide has been developed and recommended for adoption to aid law enforcement officers in assessing the dangerousness of mentally ill persons.