As a string of gun rampages continues in America and beyond, more evidence is emerging that copycat mass shooters are on the rise—a danger being amplified and accelerated by social media. Two mass shootings this month build on disturbing patterns seen in other recent cases: an attack on police in Baton Rouge, Louisiana, and another on mall goers in Munich, Germany, whose perpetrator displayed a host of behaviors underscoring this troubling phenomenon. The attacker in Baton Rouge, a 29-year-old black Army veteran who killed three cops and wounded three others before a SWAT officer took him out, was prolific on social media before he struck.