The investigation will certainly take place and things will be more clear after that but My impression is that he killed those 26 people only due to his mental illness. Just google his image and you will see a person that is depressed and anxious. His look is quite strange that’s why we can say that he can be mentally ill. Search some more images for mentally ill people and try to observe their faces. You will feel and find some similarities. So I think the only thing behind this mass shooting is mental illness. You asked me so this is my answer or opinion at the moment. President Trump also said this and I agree with his statement about his mental instability.
