From what I've learned and understand about that incident, there isnt really a real proof about the mastermind but the suspect is Ferdinand Marcos or is either his wife, Imelda Marcos
additional: I totally agree with the first answer but for me, when I look closely, Imelda is the mastermind since I've heard that Imelda is mostly one who changes Marcos' decision quickly. then Marcos then orders a few men in shooting Ninoyn
