As mass shootings have become ever more familiar, experts have come to understand them less as isolated expressions of rage and more as acts that build on the blueprints of previous rampages.

Experts in violence prevention say that many, if not most, perpetrators of such shootings have intensively researched earlier mass attacks, often expressing admiration for those who carried them out. The publicity that surrounds these killings can have an accelerating effect on other troubled and angry would-be killers who are already heading toward violence, they say.

The killing of nine people at an Oregon community college last week was a textbook example. Before opening fire, the gunman, Christopher Harper-Mercer, 26, reportedly uploaded a video about the 2012 massacre at Sandy Hook Elementary School in Newtown, Connecticut.

The perpetrator of the Sandy Hook murders was himself a student of earlier shootings — in 1999 at Columbine High School in Colorado, where 13 people were killed, and in 2011 in Norway, where 77 people were killed.

And three days after the Oregon shootings, the FBI warned colleges and universities in Philadelphia of a threat posted on the same website used by Harper-Mercer.

The potential for cultural contagion, many experts say, demands a public health response, one focused as much on early detection and preventive measures as on politically charged campaigns for firearm restrictions. But in some cases, efforts to identify and monitor potentially violent people can raise concerns about civil liberties.

"You’re balancing public welfare and personal privacy," said J. Reid Meloy, a forensic psychologist in San Diego who consults on threat assessment for schools and corporations.

Some people have also suggested changes in the way the news media covers mass attacks.

"If you blast the names and faces of shooters on news stations and constantly repeat their names, there may be an inadvertent process of creating a blueprint," said Dr. Deborah Weisbrot, an associate clinical professor of psychiatry at Stony Brook University, who has interviewed hundreds of mostly teenage boys who have made threats.

Experts emphasize that many factors, including mental health issues, may motivate a mass killer.

But anyone interested in the mechanics of such killings can reconstruct them easily through a quick Internet search of news reports, websites and social media. One website lists rampage killings around the world. The gunman who killed 12 people at a movie theater in Aurora, Colorado, in 2012 had a fan club on Tumblr.

"You’d have a hard time finding someone who didn’t do some research about those who went before," said Robert A. Fein, a psychologist whose specialty is targeted violence and an author of a 2002 report by the Secret Service on school shootings.

In a study of nine school shootings in Germany, Meloy and his colleagues found that a third of the killers had "consciously imitated and emulated what had happened in Columbine." Other mass killers have visited Columbine or written online of their admiration for the two perpetrators there.

It is easy to see why Harper-Mercer might have identified with the Sandy Hook shooter. Both young men lived with their mothers, with whom they shared a passion for guns and even went to firing ranges to shoot. Harper-Mercer’s mother said he had Asperger’s syndrome; the Sandy Hook killer had received a similar diagnosis.

"The more they identify with the characteristics of the story, the more it will increase their level of risk," said J. Kevin Cameron, director of the Canadian Center for Threat Assessment and Trauma Response, who has consulted on school shootings in the United States.

At least one study suggests that mass killings, like teenage suicides, may "cluster," with one highly covered case quickly followed by others. In a recent analysis of hundreds of killings from 1997 to 2013, researchers found that the probability of another attack was highest in the two weeks after a killing hit the news.

Some in law enforcement have begun to suggest that the news media adopt standards in reporting about such events that are similar to guidelines already in place for reporting on teenage suicides. Pete Blair, director of the Advanced Law Enforcement Rapid Response Training Center at Texas State University, has started a campaign, endorsed by the FBI, called "Don’t Name Them" — a policy that Sheriff John Hanlin of Douglas County, Oregon, asked the news media to follow in the Oregon case, with little success.

Meloy said that it might be more important to avoid photographs and phrases like "lone wolf" that "convey a certain cool pose to young people."

The New York Times ran a photograph of Harper-Mercer on its front page and featured it prominently online. Matthew Purdy, a deputy executive editor, said such images were not meant to glorify the perpetrators. "Our job is to explain and explore, and these images help to do that," he said.

Most mass killers "leak" their intentions, dropping hints in conversation or on social media. Harper-Mercer, for example, reportedly wrote in a blog post, "Seems the more people you kill, the more you’re in the limelight."

Parents, teachers, classmates, friends and others are in the best position to pick up on these clues, but they often dismiss or ignore them. So "see something, say something" strategies, like those developed in New York after the Sept. 11, 2001, terrorist attacks, can help encourage people to speak up, Meloy said.

Equally important is breaking down barriers among local agencies — law enforcement and mental health departments, for example — and developing a system to monitor threats and determine if the people who made them are simply troubled or "on a path to violence," experts in threat assessment said.

Several localities have adopted broad and coordinated prevention measures. In Los Angeles County, law enforcement, the county mental health department and educational institutions share information and train staff members to recognize and report worrisome behavior.

The county has intervened in numerous cases in which students had weapons and elaborate plans to use them, said Tony Beliz, a consultant to schools and corporations on violence prevention who for many years ran the mental health side of the effort. In the weeks immediately after a mass killing, they closely monitor young people they believe pose a risk.

After the Sandy Hook shootings, for example, they checked on a 16-year-old boy who liked bomb-making chemicals and who had told the county workers two years before, "I have to get rid of the bad people in this world." They also called the mother of a teenager who was fascinated by weapons and killing, had access to firearms and had extensively researched school shootings.

Yet such programs can sometimes collide with individual rights, especially when no crime has been committed.

Beliz and other experts said that mental health professionals and educators were often reluctant to share information about students or clients who exhibited worrisome behavior, under the misapprehension that privacy laws prohibit such disclosures. Part of the task, they said, is to educate teachers, principals and therapists, explaining the provisions in the laws that allow information to be shared if public safety is involved.

Beliz said there had so far been no school shootings in places where the Los Angeles program operated.

But, he added, "unfortunately, some campuses and law enforcement agencies are still in this state of denial where they don’t believe it’s going to happen in their community."

The biggest obstacle experts may have to overcome, though, is the reluctance of people to recognize and report signs that someone they know might be dangerous.

After Columbine, "we believed that the biggest problem we were going to deal with was overreaction to minor situations," Cameron said. "But the biggest problem we still deal with is underreaction to often blatant indicators that someone is moving on a pathway to violence."

© 2015 The New York Times Company