Which aspect is considered the most challenging to prove in a criminal investigation?

The most challenging aspect to prove in a criminal investigation is often related to the "why" and "how." Establishing the motive behind a crime — the "why" — can be difficult, as it requires insight into the psychological and emotional state of the suspect. Motives may not always be evident, and suspects may have varied reasons for their actions that are not easily discernible or provable.

Similarly, demonstrating "how" a crime was committed involves detailed reconstruction of events that occurred, which can be complicated by lack of evidence, witness reliability, and technical aspects of the crime scene. It often requires expert testimony and significant forensic analysis, which may not always yield clear conclusions.

Together, these two elements – understanding the underlying reasons for the crime and the specific actions taken – represent a significant challenge for investigators, as they are not just about confirming facts but also about interpreting human behavior and intentions, which is inherently complex.