One type of murder is the medical murderer, a type of serial killer who uses his or her position in the medical field and access to lethal drugs to kill victims. Generally, this type of killer is difficult to catch, as the person uses his or her position in a medical office or hospital to cover up the crime. Harold Shipman was a famous example of a medical killer; he used his position as a doctor in the U.K. to kill what is estimated to be 250 victims. His case was difficult to detect, and it was only unearthed once the daughter of one of his victims raised questions about her mother's will (as it left money to Shipman). The police investigation that followed turned up large does of a pain medicine in the victim's body that had been used to kill her. These types of killers are often only caught when medical boards, funeral homes, or related professionals raise questions about the medical professional's high death rate or other suspicious behavior after the fact.
The premeditated serial killer is a generally psychotic person who carries out his or her crime so that as little evidence is left as possible. However, the FBI has developing means of profiling killers through such clues as how the body was found (for example, was it posed?), whether the killer carried out sexual crimes against the victim, and whether the body was mutilated. Killers have modus operandi, or ways of carrying out their crimes, that provide clues to their identity, and they also have particular signatures, or ways that they try to satisfy their psychopathy, such as mutilating their victims. The killers' M.O. and signature help police catch them, as they can connect several crimes and try to predict the killer's next move.