This United States Army Military Police Corps Regiment manual, Army Techniques Publication ATP 3-39.12-1 Law Enforcement Investigations, Volume I April 2024, is intended as a guide and toolkit for military police investigators (MPIs), traffic management and collision investigators (TMCIs) and military police Soldiers conducting law enforcement (LE) and LE investigations. It also educates military police commanders and staffs on LE investigations capabilities, enabling a more thorough understanding of those capabilities. This increased understanding facilitates staff planning, resource allocation, and the ability to articulate LE investigative capabilities and requirements to supported commanders and organizations. The principal audience for ATP 3-39.12-1 is Army military police personnel, including Department of the Army (DA) civilian police conducting LE investigations activities while assigned to military police brigades, battalions, companies, detachments, and provost marshal (PM) staffs. This manual also provides military police commanders and staffs a framework and understanding of LE investigations and investigative capabilities that support decisive action.