The occurrence of multiple and mass murder in the United States has held constant during the last 20 years even though homicides and other kinds of violent crimes have been cut nearly in half since the early 1990s.
The problem, experts agree, is that society has done little to identify and interdict the actions of people who feel an explosive urge to kill on a grand scale. The proof is Friday's horrific slaying of 26 children and adults at Sandy Hook Elementary School in Newtown, Conn.
Despite a growing list of outrages committed at schools, theaters, shopping malls and other public places, America has not developed the means to interrupt its most sensational kind of killing.
From 1980 through 2010, there have been 20,223 homicide cases involving at least two victims, according to an exclusive analysis by Scripps Howard News Service of data the news organization assembled from FBI files and from individual police departments who do not report murder cases to the federal government.
The study found there have been at least 994 acts of mass murder, which is generally defined as the killing of four or more people in a single incident. That works out to about 32 acts of mass murder each year during the last 31 years.
"Unfortunately, we haven't been able to do a great deal to reduce mass killing. It has been virtually a straight line over time," said Jack Levin, a criminologist from Northeastern University who has extensively studied serial and mass murder.
The Scripps study found there were 29 incidents of mass murder in 2010, the most recent year for which complete data are available. That was down slightly from 37 cases in 2009 and 32 cases in 2008.
The same holds true for multiple homicides, which average about 652 cases a year. In 2010, there were 672 killings of at least two people, which were down from the 737 such cases reported in 2009.
This steady beat of multiple murders stands in stark contrast to a dramatic decline in all homicides. A significant reduction in fatal domestic violence and other disputes between acquaintances has caused annual homicides to drop from its all-time high of 24,703 killings in 1991 to 12,706 killings in 2011, according to the latest U.S. Justice Department estimates.
"Most homicides occur over an argument of some sort," said Levin. "But most mass killers -- especially those who commit random massacres at schools or shopping centers -- are methodical, well-planned and are strangers to their victims. The motivations are quite different."
The Scripps study found that 73 percent of all mass murders were committed by males, 6 percent by females and 21 percent by people of unknown gender because police did not catch them.
Mass murderers are considerably more likely to be white males than are killers in general. Sixty percent of mass murders are committed by whites, compared to 49 percent of all killings. Blacks account for 36 percent of mass killings, but 49 percent of all homicides. People of other races account for 4 percent of mass killings and 2 percent of all homicides.
Mass killings usually involve the use of firearms, especially handguns and rifles, or acts of arson, which also can kill large numbers of victims. Single-victim murders also frequently involve firearms but also include significant numbers of knifes or so-called "personal weapons" such as the killers' hands for strangulation or beating.
Generally, mass killers do not personally know their victims. Single-victim killers are more likely than mass killers to take the lives of their friends, acquaintances, wives and girlfriends, often in acts of passion involving arguments.
But mass killers sometimes do include family members among their victims. They are more likely than most murderers to take the lives of their mothers, fathers, sons or daughters. As was the case last week in Connecticut, there were at least eight cases in which mass killers included their mothers among their victims.
The study found that, generally, mass killing has held constant over time. There was one significant decline, however, in 2001 when only 19 mass killings were reported.
"After the 9/11 terrorist attacks, there was a period when we saw no school massacres," Levin said.
The traumatic impact of the 3,000 deaths in New York and Washington, D.C., changed the tone of the public discussion about death and killing. It was focused upon international terrorism rather than domestic violence.
Levin said mass killers are often copycat killers who feed on the public outrage over domestic killing.
"The copycat phenomenon is real and, unfortunately, these things come in clusters," Levin said.
The Scripps Howard study is based upon information it has assembled during the last five years from the FBI's Supplemental Homicide Report, a voluntary reporting program, and from Freedom of Information Act requests by the news service to obtain information about more than 17,600 homicides that were not reported to the federal government.