A word of caution News organizations need to pay attention to the choice of words while reporting on such incidents, says Loren Coleman, author of the book "The Copycat Effect: how the media and popular culture trigger the mayhem in tomorrow's headlines." According to Coleman, one should not, for example, report on "successful" gun attacks or "failed" suicides.
Stereotypes such as "the boy next door" or "loner" should also be avoided, he adds.
Towers' team determined that mass killings - events with four or more deaths - created a "contagious" period that lasted an average of 13 days.
One attack at a school seemed to spark the idea in other vulnerable youth.
There is a possibility that the Norwegian was a role model for the Munich killer.