For example, if the control system can be adapted to be driven very remotely (think US military drones -- operator sits in an office building in the US, drone is flying around the middle east) then there's no reason this couldn't be perpetrated by someone very far away, potentially outside of the jurisdiction of the country they're unleashing their drones on. They could go and set up the base stations months in advance of an attack in relatively remote areas, as little drone sleeper cells. When the attack happens, the perpetrators are far away, and much harder to stop.
I expect we'll see more of these, but I hope that this attack made it very clear that there needs to be more effective ways to track and disable rogue drones. I do wonder how they finally found these two (and if they are even the actual perpetrators) but I think they're gonna want to keep those cards close to their chest.