Crimes of violence on the streets are dynamic events during which participants and evidence move and change positions, witnesses watch some of the action, and suspects often flee, and police investigators arriving hours after a crime has occurred have to rely on hearsay evidence and must expect a spoiled crime scene. A patrol-based field investigator, often arriving moments after a crime has occurred, instills high levels of confidence in a courtroom and in criminal proceedings. Mobile crime scene investigators help first responders determine the limits of the evidence field and ensure that streets are blocked, backdrops are covered, and the area is not contaminated. Such early-arriving investigators can express much more confidence in their measurements, photographs, and scene reconstructions than can an investigator who arrives later when a crime scene is more likely to be compromised and tainted. The article suggests that a patrol-based crime scene technician also has an advantage in that he or she knows the actual circumstances, such as rain, snow, or wind, under which the scene was processed. Conscientious agencies should attempt to deploy crime scene investigation equipment and personnel in such a way that data collection may begin within minutes, not hours, of a crime occurring. This article suggests that patrol-based field investigators are a simple means for securing a crime scene quickly.