The United States is a less violent country than it was two decades ago. The homicide rate, which hit a peak in the early 1990s at about 10 per 100,000 people, has been cut in half, to a level not seen since the early 1960s.

But there has been no corresponding decline in mass murder — these sudden, stunning eruptions of violence with multiple victims, often perpetrated by gunmen whom researchers refer to as “pseudo-commandos.” Such a killer, clad in body armor and with a small arsenal of firearms, struck Friday in Aurora, Colo., leaving a dozen dead, 58 wounded and a nation horrified.

And mystified: Why does this happen? How can we stop it?

The statistics on mass murder suggest it is a phenomenon that does not track with other types of violent crime, such as street violence. It does not seem to be affected by the economy or by law enforcement strategies. The mass murderer has become almost a stock figure in American culture, someone bent on overkill — and, so often, seemingly coming out of nowhere.

The United States experienced 645 mass-murder events — killings with at least four victims — between 1976 and 2010, according to Northeastern University criminologist James Alan Fox. When graphed, these incidents show no obvious trend. The numbers go up and down and up again. The total body count: 2,949.

Beyond the raw data, there is the psychic toll. Mass murder, when amplified in the news media, turns a big country very quickly into a small one and turns faceless body counts into real people enduring real pain and real tragedy.

For some, the news bulletins from Aurora brought back difficult memories.

“This is personal to me,” said Rep. Carolyn McCarthy, a New York Democrat whose husband was murdered in 1993 by a spree killer on the Long Island Rail Road and who is pushing legislation to ban large ammunition magazines such as the type used in Colorado. “When something like this happens, it brings us back to a terrible day that we can’t help but remember. It’s just something that will never, never go away,” she said.

Aurora’s nightmare defies comprehension, not only because of the scale of the carnage but also because so far there is no clear motive. Many killing sprees are driven by grudges or a desire for revenge. The victims are bosses, co-workers, family members or fellow students, as was the case at Virginia Tech in 2007 and Columbine High School in 1999. Some attacks are driven by a political ideology and can be properly described as terrorism.

At the moment, the Aurora massacre remains inexplicable even by the standards of other mass killings.

“It looks about as a senseless as you can get,” said Gary LaFree, a criminologist at the University of Maryland who tracks terrorist attacks. “It is spooky.”

The issue of motive may become clearer when authorities release more information about the suspect in custody, 24-year-old James Holmes. He said nothing in his one appearance in court, looking groggy and dazed. His classmates did not know him well. Any psychiatric history is unknown.

There is a Hollywood-violence element to this latest mass killing. The Aurora killer picked a movie theater and a midnight screening of the summer’s most hyped movie, “The Dark Knight Rises.” The closest thing to a narrative explaining why Holmes would have targeted the theater is that, according to law enforcement sources, he told police he was the Joker, the Batman arch villain.

That detail has not been officially confirmed. But Randolph Roth, a professor of history and sociology at Ohio State University and author of “American Homicide,” said that if the Joker angle is true and the killer was emulating some kind of fictionalized violence, the case may echo John W. Hinckley Jr.’s attempt to assassinate President Ronald Reagan in 1981. Hinckley was fixated on the movie “Taxi Driver.”

If Holmes is “living inside the Batman movie the way Hinckley was living inside ‘Taxi Driver,’ ” Roth said, “this makes perfect sense to him even though it makes no sense to us.”

Some researchers have questioned whether violent video games play a role in mass murders, but there is no evidence for such a link, said Christopher Ferguson, a forensic psychologist at Texas A&M International University.

Mass homicide is not a new phenomenon in this country. Criminologists speak often of Howard Unruh’s rampage through the streets of Camden, N.J., in 1949 and Charles Whitman’s sniping from a tower at the University of Texas in 1966.

Mass killers are known to copy their predecessors, according to Fox. The Aurora massacre, for example, has elements in common with the pseudo-commando attack last summer in Norway.

Anders Behring Breivik first detonated a fertilizer bomb in Oslo — emulating American terrorist Timothy McVeigh’s 1995 bombing of the federal building in Oklahoma City — killing eight people. Breivik then went to an island full of young campers and, one by one, shot and killed 69 of them.

Law enforcement authorities say Holmes crafted a similarly elaborate scheme. First, authorities say, he booby-trapped his apartment with explosives. Then, they say, he used a timer to blare music starting Thursday night at midnight, just as the Batman movie was starting. That could have been an attempt to lure someone into the apartment and trigger an explosion — a diversion that would occupy police before the attack on the theater.

Even with the lower homicide rate in the United States, Americans are slain at a much higher rate than citizens in other developed nations. The U.S. homicide rate of roughly five per 100,000 people is about three times that of Canada, about four times that of Australia, nearly five times Britain’s rate and about 12 times the rate in Japan. The U.S. rate is also roughly five times that of China.

Mass murder does occur in these other countries, and it sometimes takes a bizarre form. In China, there has been a rash of killing sprees in recent years in which knife-wielding men attacked schoolchildren.

But advocates for tighter gun laws say the lethality of American violence is increased by easy access to weapons of a type originally designed for military use.

“The body armor, the large clips, the semiautomatic weapons, were all invented for military purposes to commit mass homicide,” said Roth, the Ohio State professor, who would like to see a ban on semiautomatic weapons capable of firing up to 60 rounds a minute.

Ferguson says firepower matters when it comes to mass murder. “It’s kind of hard to be a pseudo-commando with a musket in the 18th century,” he said.

One man rattled by the Aurora news was Rep. Ron Barber of Arizona, who was shot in the face and groin during the mass shooting in Tucson in January 2011. In June, Barber won the seat that had been held by his former boss, Rep. Gabrielle Giffords, who was shot in the head during the attack and who resigned to focus on her recovery.

Barber said he did not want to discuss gun laws. “The only thing I can think of right now is comforting those people who lost loved ones and who are still recovering,” he said.

Fox, the Northeastern University criminologist, said restrictive gun laws would be unlikely to stop people who are resolved to kill.

“Mass killers are extremely deliberate and determined and, no pun intended, dead set on murders,” said Fox, whose books include “Extreme Killing: Understanding Serial and Mass Murder.” “They will find the weapons they need regardless of what impediments we put in front of them. It’s not an impulsive act.”

Holmes had no known criminal history and until recently had been a graduate student studying neuroscience on a National Institutes of Health grant at the University of Colorado at Denver. Though unusually quiet and a loner, he did not strike his fellow students as a threat to society. He was facing failure for the first time, however — he had done poorly on a year-end test and had decided to drop out of school.

The psychological profile of a mass murderer generally fits someone who is suicidal, said Roger Lane, professor emeritus of history at Haverford College. And that is exactly what mass murder is, he said: a form of suicide.

“This guy knows he’s not going to get away with it,” Lane said. “None of them get away with it.”