To analyze mass killings, USA TODAY used the FBI's definition: four or more killed, not including suspects, in an event. The killing may stretch over a day or more and some distance, especially if it includes killings committed in flight or against targeted people. It does not include an extended "cooling-off period" to distinguish this kind of crime from the acts of serial killers.

Unlike gun control advocates who just count shootings, USA TODAY analyzed all mass killings, regardless of weapon. That adds significant diversity to the types of killers and victims and produces a fuller portrait of this type of crime.

USA TODAY began by collecting the FBI's Supplemental Homicide Reports for 2006-11. This data, created by local police and collated by the FBI, provide details on each murder. However, Florida does not report to the FBI; the District of Columbia and Nebraska only started doing so in 2009. And murders on American Indian reservations, college campuses and military bases may not be included. To fill these gaps, USA TODAY used local news reports and official records. USA TODAY did the same for mass killings in 2012-13, for which FBI data has not been released.

USA TODAY cross-checked each FBI report with local news reports and sometimes with local law enforcement agencies. USA TODAY found that 36 mass killings reported in the FBI data were erroneous. In some, other types of crime were miscoded as murders. In others, miscodes created a mass killing where no crime had occurred. USA TODAY excluded these from its analysis.

USA TODAY also found 26 mass killings not recorded in FBI data. Sometimes, a killer’s victims, separated by a few miles or hours, were reported as separate cases. Example: An Alabama man killed his mother and torched her house. He drove 12 miles to his uncle’s house, where he sprayed the front porch with gunfire, killing five and wounding one. Then he killed his grandmother, who lived next door, as she stood in a doorway. As he fled in a car, he shot eight more, three fatally, before killing himself.

Finally, USA TODAY included several other cases not reported by the FBI, including:

One on a U.S. military base. (USA TODAY did not include events on overseas military bases, to distinguish mass killings from terrorist attacks).

One that occurred in Sharonville, Ohio, but the killer was prosecuted in Mexico.

One in international waters off Florida. It involved U.S. citizens aboard a U.S.-based fishing vessel. It was investigated by the FBI and prosecuted in U.S. courts.

One that occurred in a national forest.

USA TODAY did not include events if deaths stemmed from negligence, such as drunken driving, even if someone was convicted. Such cases involved crimes but showed no intent to commit a mass killing. Example: Two Pittsburgh women pleaded no contest to charges stemming from a house fire that broke out where they had left two 8-year-olds in charge of three younger children. All five died.

Other classifications:

Weapons: USA TODAY counted just those a killer had “at hand” -- carried in the act. USA TODAY did not include weapons left in a vehicle or at home.

Relationships: For more than 1,000 victims, USA TODAY determined the relationship to the killer through news accounts and police and court documents. In some cases, it was impossible to tell how close a killer might have been to some types of victims, such as a family friend, co-worker or tenant. USA TODAY grouped relationships into four broad categories based on social and family distance.

USA TODAY continues to add new cases and to update old ones.