Mass killings, such as Friday's shooting in Newtown, Conn., have shocked the nation, but criminologists say to expect more.
Shooting sprees are on the rise and probably won't be subsiding any time soon.
"There's clearly been a major upswing," criminologist Gary LaFree of the University of Maryland told The Huffington Post on Friday.
How much of an increase is subject to debate. According to FBI statistics, the trend is inching upward. People killed in clusters of four or more averaged 163 annually between 2006 and 2008, just two more than back in the 1980s.
"Homicide trends don't look like EKGs," said LaFree, who is also director of the National Center for the Study of Terrorism and Responses to Terrorism. "They go up for a while and tail off for a while."
But, LaFree added, "What's interesting is that we're getting a specialized strain of violence when other forms are in decline."
One-on-one gun homicides have dropped more than 40 percent since 1980, according to 2010 FBI crime data. The firearm homicide rate sank from 10,000 in 2005 to 8,776 in 2010.
But shooting sprees are "not decreasing, because the motive in mass murder is so different from the motive for single-victim murders," Jack Levin, a criminology professor at Northeastern University and author of "Extreme Killing," told HuffPost in a previous report. "These are well-planned crimes ... Mass killings don't depend on any given time."
LaFree noted that one-on-one violence often erupts spontaneously and shows more of a correlation to economic conditions and drug circulation than mass murder.
Perpetrators behind large-scale homicides often choose assault weapons that fire multiple rounds for maximum carnage. "If those weren't available, it's hard for me to imagine that these mass killings wouldn't be reduced," criminologist Daniel Nagin of Carnegie Mellon told HuffPost on Friday. But, he quickly pointed out, whether stricter laws would be effective in doing so is a separate issue.
LaFree said gun control measures would probably have a greater effect on anger-ignited handgun violence than on killers hellbent on inflicting mass casualties.
According to a Scripps-Howard study of FBI statistics, 4,685 people died in 965 mass-murders between 1980 and 2008. A USA Today article put the yearly average of incidents at about about 20 a year.
A quick look at some of the more notable multiple slayings this year doesn't seem to point to a huge spike. Incidents in Aurora, Colo., Oak Creek, Wisc., Oakland, Calif., Portland, Minneapolis, Milwaukee, and now Newtown add up to less than half of the average 163 annual victims noted above. But the quick tally does not account for every clustered killing, and data-collecting can vary (through the years and by agency, too).
Still, the numbers LaFree has seen moved him to offer a grim forecast.
"My guess is that we're witnessing a phenomenon that we're likely to see for the forseeable future," he said.
Clarification: Language has been adjusted to reflect the fact that most recent killings have not involved fully automatic weapons.