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Kevin A. Kepple and Jeff Dionise, USA TODAY

Meghan Hoyer and Brad Heath, USA TODAY

Mass killers target Americans once every two weeks on average, in attacks that range from robberies to horrific public shooting sprees like the massacre Friday of 27 people in Newtown, Conn., a USA TODAY examination found.

Using news accounts and FBI records from 2006 through 2010, the most recent years for which complete records were available, USA TODAY identified 156 murders that met the FBI definitions of mass killings, where four or more people were killed.

All told, the attacks killed 774 people, including at least 161 young children.

The review offers perhaps the most current, complete picture yet of a crime that is both frighteningly common and not widely understood.

"Everybody is surprised when they hear it's dozens a year," said Northeastern University criminologist James Alan Fox, who has studied mass murders. "People don't understand them. When they think of mass murders, they only think it's random."

USA TODAY's examination did not include murders during the past two years, both of which were marked by a series of high-profile public shootings, including a rampage this year at an Aurora, Colo., movie theater that left 12 dead and 57 injured, and an attack on a Sikh temple in Wisconsin that killed six.

Without more complete records, it is impossible to know whether mass killings increased over those years - though they have become less common since the mid-1990s, according to Grant Duwe, director of research at the Minnesota Department of Corrections, who has studied mass murders.

The killings between 2006 and 2010, however, offer a portrait of mass murder that in many ways belies the stereotype of a lone gunman targeting strangers:

- Lone gunmen, such as the one who terrorized Sandy Hook Elementary School last week, account for less than half of the nation's mass killers. About a quarter of mass murders involve two or more killers.

- A third of mass killings didn't involve guns at all. In 15 incidents, the victims died in a fire. In 20 others, the killer used a knife or a blunt object. When guns were involved, killers were far more likely to use handguns than any other type of weapon.

- Children are frequently victims. At least 161 who died in mass killings -- roughly one in five -- were 12 and younger.

- Mass murderers tend to be older than other killers, with an average age of nearly 32 years old. Like all killers, they are overwhelmingly men.

Friday's massacre in Newtown "has turned a whole new page" in the nation's long-running debate over guns, said Rep. Carolyn McCarthy, D-N.Y., a leading proponent of tighter gun laws. "Parents and grandparents, dads, gun owners are thinking that their children at any time at any place could have someone come in and do this kind of massacre."

But for all the attention they receive, mass killings still accounted for only a tiny fraction - about 1% - of all the Americans who were murdered over those five years. During those five years, more died from migraines and falling out of chairs than were murdered by mass killers, according to death records kept by the U.S. Centers for Disease Control and Prevention. Three times as many people perished from sunstroke.

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