The Australian suspect who claimed responsibility for the New Zealand shootings that killed at least 49 people in two mosques wrote a 70-page anti-immigrant manifestocalling himself a racist, ethno-nationalist and fascist. He also called President Trump a symbol of "renewed white identity." Trump has condemned the attack.

(In 2017, a man in Quebec killed six people at a mosque and cited similar influences.)

In a less publicized shooting this week, two young men killed seven people at a school in Brazil. Police say they were obsessed with the 1999 Columbine attack.

(Shooters in Germany, Canada and Finland have also cited Columbine.)

Mass shootings are often called a uniquely American problem, but experts say violence here has global impact.

"There's no doubt that previous mass shooters in the United States have been imitated by shooters in the U.S. and outside it," said Adam Lankford, a criminal justice professor at the University of Alabama who studies mass shootings, noting "the influence of America culturally and cross-culturally."

The United States is ranked No. 1 in international influence, based on its economic and military power as well as its "cultural imprint," according to U.S. News & World Report.

"America’s culture and actions are contagious, as the U.S. draws a lot of attention," said Gary Slutkin, founder of Cure Violence, a nonprofit which treats gun violence like a disease.

Though the U.S. has long held a position of cultural dominance in the world, news of American mass shootings — and the motivations and ideologies behind them — spread faster and farther than ever. CNN International is seen in more than 200 countries and territories worldwide. And if it weren't on TV, it would still be on Twitter.

"It's the viral nature of school shootings, the ubiquity of technology and the spread of ideology ... that have made mass shootings a global problem," said Colin Clarke, an adjunct political scientist at the RAND Corporation and a senior research fellow at the Soufan Center, which does research on human security. "But while we pay a lot of attention to jihadist terrorism, we've been very slow and stubborn to realize that right-wing terrorism is very global, too."

Social media amplifies hate, said Richard Cohen, president of the Southern Poverty Law Center, which tracks hate groups.

“The atrocity in New Zealand shows us, once again, that we’re dealing with an international terrorist movement linked by a dangerous white supremacist ideology that’s metastasizing in the echo chambers of internet chat rooms and on social media networks," he said.

The mosque shooting suspect used a helmet-mounted camera to capture footage of the killings which he streamed live on Facebook and posted on YouTube and Twitter. It has since been removed.

An extremely small percentage of people commit mass shootings, but it is in these spaces online, Lankford said, where toxic ideas spread, and where many alienated men feel safe to worship the gunmen who terrorize the rest of us. Lankford says he's presenting a new study at the National Science Foundation next month showing that since 2010 there's been a more than 80-percent increase in highly lethal mass shooters that were influenced by a previous attacker.

A 2016 paper presented at the American Psychological Association’s annual convention blamed “media contagion” for an increase in mass shootings. The authors wrote: "We would argue identification with prior mass shooters made famous by extensive media coverage, including names, faces, writings, and detailed accounts of their lives and backgrounds, is a more powerful push toward violence than mental health status or even access to guns."

Indeed, the majority of gun deaths in the U.S. are due not to mass shootings but to suicide, which also has a contagion effect.

Mass shooting contagion is particularly heightened for 13 days after an event, according to a 2015 analysis by researchers at Arizona State University. Some shooters, however, spend much more time planning their attacks. The New Zealand shooter claims he planned for two years.

Evidence shows mass shootings often have strong copycat effects, particularly for a certain kind of offender, said Jonathan Metzl, a professor of sociology and psychiatry at Vanderbilt University and author of Dying of Whiteness: How the Politics of Racial Resentment Is Killing America's Heartland.





"The thing that's being exported is not just mass shootings, it's a particular form of hate and hate crimes that blames immigrants and outsiders and people who look different," Metzl said. "It's impossible to see this crime and this mass murder just as a mass shooting. It took place in the context of the global spread of white nationalism."

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Contributing: Cara Kelly, USA TODAY