Dangerous Serial Killers Who Were Never Caught
A serial killer is a person who murders three or more people, usually due to abnormal psychological gratification, with the murders taking place over more than a month and including a significant break between them. Some sources, such as the FBI, disregard the three or more criterion and define serial killing as a series of two or more murders, committed as separate events, usually, but not always, by one offender acting alone.
Although psychological gratification is the usual motive for serial killing, and most serial killings involve sexual contact with the victim, the FBI states that the motives of serial killers can include anger, thrill, financial gain, and attention seeking. The murders may be attempted or completed in a similar fashion and the victims may have something in common: race, appearance, sex, or age group, for example.
Serial killing is not the same as mass murdering, nor is it spree killing, in which murders are committed in two or more locations in a short time. However, cases of extended bouts of sequential killings over periods of weeks or months with no apparent cooling off period or return to normalcy have caused some experts to suggest a hybrid category of spree-serial killer. (source : wikipedia)