What Was Matt's Mistake

Essentially, everything he did was a mistake. Bombing scenes are basically no different than other crime scenes in basic processing principles and procedures, many times on a larger scale Basic principles such as scene safety and security, scene documentation, evidence collection/processing, specialized equipment, witness and victim interviews, removal of human remains, and releasing the scene are all based on the same principles.
Matt’s first mistake was “immediately” going straight to the seat of the blast (not setting up a perimeter and safety/security etc. aside). He may have destroyed evidence by walking on it, contaminated the scene by bringing in foreign material and cross contaminating, contaminated himself with possible toxic or