A serial killer is typically a person who kills three or more people, with the murders taking place over more than a month and including a significant period of time between them. The Federal Bureau of Investigation (FBI) defines serial murder as "a series of two or more murders, committed as separate events, usually, but not always, by one offender acting alone".The United States has by far the largest number of documented serial killers in the world. According to Radford University's Serial Killer Information Center, it has more documented serial killers than the next ten highest countries on the list.