It's hard to say. Maybe he left a manifesto or something behind? And what were the "explosives" in his vehicle? Fireworks? Or something else? If they were bombs or bomb-making materials, why did he have that and what was he planning to do with it?
I saw one story yesterday with a witness named "Greg" who was interviewed afterwards. He claimed that he and others in his group saw the shooter climbing up the building and standing on top with a firearm. They tried to get law enforcement's attention but for whatever reason they were ignored, according to "Greg." Let me see if I can find that story.