Mass killings, such as recent shootings at a college in southern Oregon, a prayer meeting in Charleston, South Carolina, or the 2012 attack at Sandy Hook Elementary School in Newtown, Connecticut, have become increasingly common across the United States.

Now, new findings published in the journal PLOS ONE affirm what law enforcement officials and gun control advocates have said for years: Mass shootings are “contagious” and often inspire copycat attacks, NBC News reports.

For the study, researchers at Arizona State University and Northeastern Illinois University reviewed databases maintained by the Brady Campaign to Prevent Gun Violence. In addition, scientists examined reports from the FBI to evaluate the latest statistics about shootings in the United States.

Their findings showed that, on average, mass killings involving firearms occurred almost every two weeks in the United States and school shootings occurred about once each month.

Researchers also found that up to 20 to 30 percent of mass killings in this country were triggered by previous attacks. Scientists said the “copycat effect” from these violent episodes lasted about 13 days. The report also mentioned findings of several past studies revealing how news reports about suicides and homicides can plant ideas into the heads of “at-risk individuals to commit similar acts.”

The analysis further stated that mass killings were far more common in states where more people owned guns.

Did you know that injuries and violence are the leading killers of Americans age 30 and younger? Click here for more information.