I think may be white extremist group using this to push their insane agenda (anti govrnment, etc etc) under cover of racially charged killings in last few days.....At the same time shooting into the crowd full of minorities. If the shooters are black, and motivated by the recent incidents, (or pushed, so to speak) they had 2 days to recruit, plan, equip, and execute, could include making explosive devices. I think that sounds off. A group of black men and women planning a mass killing would be near unprecedented. If they are white extremists, already part of a group, they may have been planning something already, and used this as a shield. Perhaps, thinking they would get away.
The easiest answer though is direct response to recent killings.....That may be the case. ISIS/related sounds off too, because their attacks have been way softer, large groups of innocent people, high body counts. This is targeted against polices/and or protestors, but they could have killed dozens elsewhere.