To the FBI, serial murder means two or more victims in separate events

Where did the FBI’s definition of serial murder come from?

It came from a symposium on serial murder that the FBI hosted in San Antonio, Texas, in 2005. The definition was crafted by combining the ideas put forth there. The FBI’s report says a lower number of victims would allow law enforcement more flexibility in committing resources to a potential serial murder investigation.^1,3,6,12

In 2005 the FBI hosted a symposium on serial murder in San Antonio, Texas.^1,12 The FBI’s report on the symposium says previous definitions had specified anywhere from two to ten victims, and the attendees examined the variations in order to develop a single definition.²

Academics and researchers wanted a specific number of murders, to allow clear inclusion criteria for their research.³ The definition was meant for law enforcement, and a lower number of victims would allow law enforcement more flexibility in committing resources to a potential serial murder investigation.³ Motivation was left out because the attendees felt it would make the definition overly complex.³

The symposium’s discussion groups agreed on factors to include in a definition, among them two or more murdered victims and incidents occurring in separate events, at different times.⁶ They also agreed that the time period between murders separates serial murder from mass murder.⁶ The definition crafted from those ideas is one sentence: “The unlawful killing of two or more victims by the same offender(s), in separate events.”⁶

What did federal law say before the FBI’s definition?

Congress put a definition of serial killings into federal law in 1998.^8,10 The Protection of Children From Sexual Predators Act of 1998 added section 540B, “Investigation of serial killings,” to Title 28 of the United States Code.^9,10 Section 540B defines serial killings as “a series of three or more killings, not less than one of which was committed within the United States, having common characteristics such as to suggest the reasonable possibility that the crimes were committed by the same actor or actors.”^9,10

Section 540B lets the Attorney General and the FBI Director investigate serial killings that violate state or local law, if the head of a law enforcement agency with jurisdiction over the offense requests it.^9,12 The FBI’s report says the definition is limited in its application: its purpose was to set criteria for FBI assistance to local law enforcement agencies in serial murder investigations.⁸ In the report’s words, it “was not intended to be a generic definition for serial murder.”⁸

How do spree murder and mass murder differ from serial murder?

Most earlier definitions required a period of time between the murders, described as separate occasions, a cooling-off period or an emotional cooling-off period.⁷ The general definition of spree murder is two or more murders by an offender or offenders without a cooling-off period, and the lack of one marks the difference from serial murder.⁴

At the symposium, the validity of spree murder as a separate category was discussed at great length, and the definitional problems of the cooling-off period were central to the discussion.⁵ Because the concept creates arbitrary guidelines, the majority of attendees advocated disregarding spree murder as a separate category.⁵

The FBI’s report says mass murder was generally described as four or more murders occurring during the same incident, with no distinctive time period between the murders.^7,12 Federal law set a lower line in 2013.^11,12 Congress defined “mass killings” as “3 or more killings in a single incident” in the Investigative Assistance for Violent Crimes Act of 2012, enacted January 14, 2013.^11,12

Like the 1998 definition, the 2013 one is written for federal help.^8,11 It applies for the purposes of a provision that lets the Attorney General, at the request of a state or local law enforcement official, assist in investigating mass killings and attempted mass killings.^11,12

Why is the Moscow, Idaho case not serial murder?

Madison Mogen, Kaylee Goncalves, Xana Kernodle and Ethan Chapin were killed in the same home in Moscow, Idaho, in the early morning hours of November 13, 2022. That was one incident. The FBI’s definition of serial murder requires separate events. Federal law defines three or more killings in a single incident as mass killings.^{6,11,13,14,16}

Madison Mogen, Kaylee Goncalves, Xana Kernodle and Ethan Chapin were University of Idaho students.^13,14 They were killed in the same home in Moscow, Idaho, in the early morning hours of November 13, 2022.^13,14,16

Bryan Kohberger pleaded guilty to four counts of first-degree murder and one count of burglary on July 2, 2025.^13,15,16 On July 23, 2025, a judge sentenced him to four consecutive life terms without the possibility of parole, and 10 years for burglary.^14,15

Four killings in one incident do not meet the FBI’s definition of serial murder, which requires separate events.⁶ They meet the federal definition of mass killings: three or more killings in a single incident.^11,12 Under those definitions, the case is a mass killing, not a series.