A serial killer is someone who commits three or more murders over an extended period of time with cooling-off periods in between.
In between their crimes, they appear to be quite normal,this is called the "mask of sanity."
There is frequently but not always a sexual element to the murders.
A mass murderer, on the other hand, is an individual who kills three or more people in a single event and in one location.
The perpetrators sometimes commit suicide, meaning knowledge of their state of mind and what triggers their actions is often left to more speculation than fact.
Mass murderers who are caught sometimes claim they cannot clearly remember the event.
A spree killer commits multiple murders in different locations over a period of time that may vary from a few hours to several days.
Unlike serial killers, however, they do not revert to their normal behavior in between slayings.
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