"Serial murder cases present numerous challenges and obstacles to law enforcement personnel who have the responsibility of investigating these complex cases. These cases involve multiple victims; the series may span days, months or even years; they can involve several jurisdictions; the motive involved may not be easily discerned; offender behaviors may not be consistent among all the cases; and there may be no obvious relationship between the offenders and the victims. Serial murder cases are also very rare and most law enforcement investigators do not have the same level of experience in investigating serial murder as they do with other types of crimes. Additionally, the majority of serial murder cases involve offenders who kill for sexual reasons. The crime scene dynamics in sexually motivated murders can appear very different from those of other violent crimes. The physical and particularly the sexual interactions committed by offenders against victims are unusual, tend to appear bizarre, and can be difficult to interpret (Morton & Hilts, 2008)."
Federal Bureau of Investigation: http://www.fbi.gov/