Trophies and souvenirs are an important part of the killer’s modus operandi (“method of operation,” or M.O.), the name given to the particular tools and strategies that distinguish the killer’s work. The M.O. includes factors such as the location of the crimes, the tools used, the time of day, the alibi, and any accomplices involved. The M.O. may, of course, evolve over time as the killer becomes more experienced. The investigators will be particularly interested in any details that are unique to that killer, such as leaving a note behind. They will also look for the signature of the crime. Trophies and souvenirs can be part of the signature, as can mutilating or having sex with the corpse, or placing the body in a certain position.