Ignoring other mass shootings that didn't use guns that look like that doesn't mean they didn't happen. According to FBI statistics, nearly all mass-shootings occur with run of the mill handguns and I believe part of the reason AR-15's are becoming more common to use by mass-shooters is because the Media is going around, ranting for days about what effective killing machines they are and exaggerating how much more effective than other guns they are.

On another note, if the media really care about preventing mass-shootings, they should show it by refusing to show the pictures of or give the name of the shooter, as studies show that giving them infamy and attention attracts copycats looking for attention and infamy