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For me, the most significant aspect of this whole incident is his motivation, especially considering the time he invested in it. He went to a LOT of trouble to plan and set this up, over a long period of time. That tends to rule out "trigger" events. ISIS involvement seems unlikely; if he'd turned into an Islamic extremist of this degree, they would at least have found a Koran lying around. Another possibility is a psychiatric disorder, where over time he developed a delusional, paranoid personality resulting from some form of schizophrenia or possibly a brain tumor. Sometimes these behaviors do develop slowly, over time. I used to work in a mental hospital and have seen many, many patients who fit this pattern.