Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

Police have said so for years and now scientists have measured the effect: Mass shootings and school attacks do inspire copycats.

As many as 20 to 30 percent of attacks are set off by other attacks, according to researchers at Arizona State University and Northeastern Illinois University. The effect lasts about 13 days, they write in the report published Thursday in the Public Library of Science journal PLoS ONE.

And mass killings — such as the 2012 attack on small children at Sandy Hook elementary school in Connecticut, the 1999 Columbine massacre, and last month’s shooting of nine people at a prayer meeting in Charleston — are becoming alarmingly common in the United States.

"On average, mass killings involving firearms occur approximately every two weeks in the U.S., while school shootings occur on average monthly," wrote Sherry Towers, a research professor at ASU, and her colleagues.

Such attacks are more common in states where more people own guns, they added in their report.

"Statistics are not readily available on the incidence of mass killings and school shootings in other industrialized countries, however studies have shown that the firearm homicide and suicide rates in the U.S. are several times higher than that of any other industrialized country and the patterns appear to be due to higher rates of firearm ownership in the U.S. compared with other industrialized countries," they wrote.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

They looked at databases maintained by the Brady Campaign to Prevent Gun Violence and reports from the Federal Bureau of Investigation to make their calculations.

“Several past studies have found that media reports of suicides and homicides appear to subsequently increase the incidence of similar events in the community, apparently due to the coverage planting the seeds of ideation in at-risk individuals to commit similar acts,” they wrote.

The word they use is contagious.

“We find significant evidence that mass killings involving firearms are incented by similar events in the immediate past. On average, this temporary increase in probability lasts 13 days, and each incident incites at least 0.30 new incidents.”

The FBI noted that mass shootings appeared to be on the rise in the U.S. in a 2014 report.

“The copycat phenomenon is real,” Andre Simons of the FBI's Behavioral Analysis Unit said at the time. “As more and more notable and tragic events occur, we think we're seeing more compromised, marginalized individuals who are seeking inspiration from those past attacks.”

Towers started her own research after one of the shootings touched on her own life.

"In January of 2014 I was due to have a meeting with a group of researchers at Purdue University," she said in a statement. "That morning there was a tragic campus shooting and stabbing incident that left one student dead," she added.

"On average, mass killings involving firearms occur approximately every two weeks in the U.S., while school shootings occur on average monthly."

"I realized that there had been three other school shootings in the news in the week prior, and I wondered if it was just a statistical fluke, or if somehow through news media those events were sometimes planting unconscious ideation in vulnerable people for a short time after each event."

News editors and reporters worry about this, also, and often discuss how to report such attacks without making them seem glamorous.

"It occurred to us that mass killings and school shootings that attract attention in the national news media can potentially do the same thing, but at a larger scale," Towers said.