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.
https://www.nytimes.com/2015/10/08/science/mass-killers-often-rely-on-past-perpetrators-blueprints.html