The FBI's "Crime Classification Manual" places serial killers into three categories: "organized", "disorganized", and "mixed" (i.e., offenders who exhibit organized and disorganized characteristics). Some killers descend from being organized into disorganized as their killings continue, as in the case of psychological decompensation or overconfidence due to having evaded capture, or vice versa, as when a previously disorganized killer identifies one or more specific aspects of the act of killing as his/her source of gratification and develops a "modus operandi" structured around those.