Mostly gang-related, although mass shooters have a tendency to be Democrats.
Via Daily Caller:
Blue states have 42 percent more mass shootings than red states after adjusting for population, according to data published by Vox, a progressive media outlet, and examined by The Daily Caller News Foundation.
Vox published its data after the Orlando terror attack last Sunday, and it suggests that blue states, which tend to have extremely strict gun laws, are ironically much more likely to have mass shootings than red states with less strict gun laws.